Document And Entity Information - USD ($) $ in Billions	12 Months Ended
Document And Entity Information - USD ($) $ in Billions	Dec. 31, 2017	Jan. 31, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	SEI INVESTMENTS COMPANY
Entity Central Index Key	0000350894
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	SEIC
Amendment Flag	false
Entity Common Stock, Shares Outstanding		157,278,314
Entity Public Float			$ 6.6
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 744,247	$ 695,701
Restricted cash	3,505	3,500
Receivables from investment products	56,666	61,761
Receivables, net of allowance for doubtful accounts of $695 and $523	282,706	227,957
Securities owned	21,526	21,339
Other current assets	31,158	27,575
Total Current Assets	1,139,808	1,037,833
Property and Equipment, net of accumulated depreciation of $309,955 and $285,322	146,428	146,190
Capitalized Software, net of accumulated amortization of $350,045 and $303,540	310,405	295,867
Investments Available for Sale	87,983	84,033
Investments in Affiliated Funds, at fair value	6,034	4,858
Investment in Unconsolidated Affiliates	59,492	50,459
Goodwill	52,990	0
Intangible Assets, net of accumulated amortization of $1,552	28,578	0
Deferred Income Taxes	2,767	2,127
Other Assets, net	18,884	15,456
Total Assets	1,853,369	1,636,823
Current Liabilities:
Accounts payable	5,268	5,966
Accrued liabilities	265,058	240,525
Deferred revenue	4,723	2,880
Total Current Liabilities	275,049	249,371
Borrowings Under Revolving Credit Facility	30,000	0
Long-term Income Taxes Payable	10,629	0
Deferred Income Taxes	48,472	69,693
Other Long-term Liabilities	12,380	14,645
Total Liabilities	376,530	333,709
Commitments and Contingencies
SEI Investments shareholders' equity:
Series Preferred stock, $.05 par value, 50 shares authorized; no shares issued and outstanding	0	0
Common stock, $.01 par value, 750,000 shares authorized; 157,069 and 159,031 shares issued and outstanding	1,571	1,590
Capital in excess of par value	1,027,709	955,461
Retained earnings	467,467	384,018
Accumulated other comprehensive loss, net	(19,908)	(37,955)
Total Shareholders' Equity	1,476,839	1,303,114
Total Liabilities and Equity	$ 1,853,369	$ 1,636,823

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current Assets:
Receivables, allowance for doubtful accounts	$ 695	$ 523
Property and Equipment, accumulated depreciation	309,955	285,322
Capitalized Software, accumulated amortization	350,045	303,540
Intangible Assets, accumulated amortization	$ 1,552	$ 0
Equity:
Series Preferred stock, par value	$ 0.05	$ 0.05
Series Preferred stock, shares authorized	50,000	50,000
Series Preferred stock, shares issued	0	0
Series Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	750,000,000	750,000,000
Common stock, shares issued	157,069,000	159,031,000
Common stock, shares outstanding	157,069,000	159,031,000

Consolidated Statements Of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Asset management, administration and distribution fees	$ 1,180,789	$ 1,072,176	$ 1,010,511
Information processing and software servicing fees	321,741	300,346	290,893
Transaction-based and trade execution fees	24,022	29,023	32,804
Total revenues	1,526,552	1,401,545	1,334,208
Expenses:
Subadvisory, distribution and other asset management costs	179,181	170,961	160,062
Software royalties and other information processing costs	31,275	30,323	31,497
Brokerage commissions	17,845	22,152	24,388
Compensation, benefits and other personnel	458,737	414,622	395,774
Stock-based compensation	36,366	16,017	17,312
Consulting, outsourcing and professional fees	186,357	166,769	146,436
Data processing and computer related	71,909	64,930	58,884
Facilities, supplies and other costs	72,352	68,245	74,968
Amortization	48,275	45,392	42,630
Depreciation	27,311	26,440	24,044
Total expenses	1,129,608	1,025,851	975,995
Income from operations	396,944	375,694	358,213
Net gain (loss) from investments	1,269	112	(456)
Interest and dividend income	7,057	4,316	3,358
Interest expense	(781)	(531)	(483)
Equity in earnings of unconsolidated affiliates	152,550	126,103	137,057
Gain on sale of subsidiary	0	2,791	2,791
Income before income taxes	557,039	508,485	500,480
Income taxes	152,650	174,668	168,825
Net income	$ 404,389	$ 333,817	$ 331,655
Basic earnings per common share	$ 2.56	$ 2.07	$ 2.00
Shares used to compute basic earnings per share	158,177	161,350	165,725
Diluted earnings per common share	$ 2.49	$ 2.03	$ 1.96
Shares used to compute diluted earnings per share	162,269	164,431	169,598
Dividends declared per common share	$ 0.58	$ 0.54	$ 0.50

Consolidated Statements Of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 404,389	$ 333,817	$ 331,655
Other comprehensive gain (loss), net of tax:
Foreign currency translation adjustments	17,597	(12,131)	(14,900)
Unrealized holding gain (loss) on investments:
Unrealized holding gains (losses) during the period, net of income taxes of $1, $457 and $822	190	(918)	(1,659)
Less: reclassification adjustment for losses realized in net income, net of income taxes of $(99), $(201) and $(76)	260	384	164
Total other comprehensive gain (loss), net of taxes	18,047	(12,665)	(16,395)
Comprehensive income	$ 422,436	$ 321,152	$ 315,260

Consolidated Statements Of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Unrealized holding gains (losses) during the period, income taxes	$ 1	$ 457	$ 822
Reclassification adjustment for losses realized in net income, income taxes	$ (99)	$ (201)	$ (76)

Consolidated Statements Of Changes In Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning balance at Dec. 31, 2014		$ 1,667	$ 834,615	$ 420,226	$ (8,895)
Beginning balance, shares at Dec. 31, 2014		166,688
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Purchase and retirement of common stock		$ (60)	(22,984)	(266,543)
Purchase and retirement of common stock, shares		(5,951)
Issuance of common stock under the employee stock purchase plan		$ 1	2,798
Issuance of common stock under the employee stock purchase plan, shares		69
Issuance of common stock upon exercise of stock options		$ 29	62,716
Issuance of common stock upon exercise of stock options, shares	2,927	2,927
Stock-based compensation	$ (17,312)		17,312
Tax benefit on stock options exercised			16,056
Net income	331,655			331,655
Dividends declared ($0.58, $0.54 and $0.50 per share)	(82,478)			(82,478)
Other comprehensive gain (loss)	(16,395)				(16,395)
Total Equity at Dec. 31, 2015	$ 1,289,720	$ 1,637	910,513	402,860	(25,290)
Ending balance, shares at Dec. 31, 2015		163,733
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect upon adoption of ASU 2016-09			0	0
Purchase and retirement of common stock		$ (66)	(28,306)	(266,002)
Purchase and retirement of common stock, shares		(6,600)
Issuance of common stock under the employee stock purchase plan		$ 1	3,357
Issuance of common stock under the employee stock purchase plan, shares		88
Issuance of common stock upon exercise of stock options		$ 18	44,896
Issuance of common stock upon exercise of stock options, shares	1,809	1,810
Stock-based compensation	$ (16,017)		16,017
Tax benefit on stock options exercised			8,984
Net income	333,817			333,817
Dividends declared ($0.58, $0.54 and $0.50 per share)	(86,657)			(86,657)
Other comprehensive gain (loss)	(12,665)				(12,665)
Total Equity at Dec. 31, 2016	$ 1,303,114	$ 1,590	955,461	384,018	(37,955)
Ending balance, shares at Dec. 31, 2016	159,031	159,031
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect upon adoption of ASU 2016-09			0	0
Purchase and retirement of common stock		$ (44)	(20,243)	(227,827)
Purchase and retirement of common stock, shares		(4,403)
Issuance of common stock under the employee stock purchase plan		$ 1	3,280
Issuance of common stock under the employee stock purchase plan, shares		71
Issuance of common stock upon exercise of stock options		$ 24	50,263
Issuance of common stock upon exercise of stock options, shares	2,370	2,370
Stock-based compensation	$ (36,366)		36,366
Tax benefit on stock options exercised			0
Net income	404,389			404,389
Dividends declared ($0.58, $0.54 and $0.50 per share)	(91,444)			(91,444)
Other comprehensive gain (loss)	18,047				18,047
Total Equity at Dec. 31, 2017	$ 1,476,839	$ 1,571	1,027,709	467,467	$ (19,908)
Ending balance, shares at Dec. 31, 2017	157,069	157,069
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect upon adoption of ASU 2016-09			$ 2,582	$ (1,669)

Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 404,389	$ 333,817	$ 331,655
Depreciation	27,311	26,440	24,044
Amortization	48,275	45,392	42,630
Equity in earnings of unconsolidated affiliates	(152,550)	(126,103)	(137,057)
Distributions received from unconsolidated affiliate	143,517	125,224	141,767
Stock-based compensation	36,366	16,017	17,312
Provision for losses on receivables	172	(126)	(135)
Deferred income tax expense	(21,046)	4,794	(1,394)
Gain from sale of SEI AK	0	(2,791)	(2,791)
Net (gain) loss from investments	(1,269)	(112)	456
Tax benefit on stock options exercised	0	8,984	16,056
Change in long-term taxes payable	10,629	0	0
Change in other long-term liabilities	(2,265)	3,248	1,070
Change in other assets	(1,489)	(1,917)	783
Write off of capitalized and purchased software	0	0	6,055
Other	(2,191)	390	(2,440)
Decrease (increase) in Receivables from investment products	5,095	(13,663)	295
Decrease (increase) in Receivables	(52,726)	(4,807)	(28,469)
Decrease (increase) in Other current assets	(3,583)	(1,368)	(8,014)
Increase (decrease) in Accounts payable	(909)	1,455	(5,441)
Increase (decrease) in Accrued liabilities	20,992	18,851	10,498
Increase (decrease) in Deferred revenue	1,180	495	636
Total adjustments	55,509	100,403	75,861
Net cash provided by operating activities	459,898	434,220	407,516
Cash flows from investing activities:
Decrease in restricted cash	0	2,000	301
Additions to property and equipment	(25,525)	(31,397)	(44,465)
Additions to capitalized software	(61,043)	(50,392)	(29,416)
Purchases of marketable securities	(69,525)	(73,193)	(52,538)
Prepayments and maturities of marketable securities	65,830	54,141	38,551
Sales of marketable securities	0	15,152	7,761
Cash paid for acquisition, net of cash acquired	(80,234)	0	0
Receipt of contingent payment from sale of SEI AK	0	2,791	2,791
Other investing activities	(1,805)	1,313	(1,000)
Net cash used in investing activities	(172,302)	(79,585)	(78,015)
Cash flows from financing activities:
Borrowings under revolving credit facility	40,000	0	0
Repayments under revolving credit facility	(10,000)	0	0
Purchase and retirement of common stock	(248,339)	(292,258)	(291,374)
Proceeds from issuance of common stock	53,568	48,272	65,543
Payment of dividends	(88,862)	(84,686)	(80,030)
Net cash used in financing activities	(253,633)	(328,672)	(305,861)
Effect of exchange rate changes on cash and cash equivalents	14,583	(9,923)	(11,425)
Net increase in cash and cash equivalents	48,546	16,040	12,215
Cash and cash equivalents, beginning of year	695,701	679,661	667,446
Cash and cash equivalents, end of year	744,247	695,701	679,661
Interest paid	699	531	460
Income taxes paid	165,049	157,255	159,605
Non-cash financing activities
Dividends declared but not paid	$ 47,179	$ 44,596	$ 42,625

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Nature of Operations

SEI Investments Company (the Company), a Pennsylvania corporation, provides investment processing, investment management, and investment operations platforms to financial institutions, financial advisors, institutional investors, investment managers and ultra-high-net-worth families in the United States, Canada, the United Kingdom, continental Europe and other various locations throughout the world. Investment processing platforms consist of application and business process outsourcing services, professional services and transaction-based services. Revenues from investment processing platforms are recognized in Information processing and software servicing fees on the accompanying Consolidated Statements of Operations, except for fees earned associated with trade execution services.

Investment management programs consist of mutual funds, alternative investments and separate accounts. These include a series of money market, equity, fixed-income and alternative investment portfolios, primarily in the form of registered investment companies. The Company serves as the administrator and investment advisor for many of these products. Revenues from investment management programs are recognized in Asset management, administration and distribution fees on the accompanying Consolidated Statements of Operations.

Investment operations platforms offer investment managers support for traditional investment products such as mutual funds, collective investment trusts, exchange-traded funds, and institutional and separate accounts, by providing outsourcing services including fund and investment accounting, administration, reconciliation, investor servicing and client reporting. These platforms also provide support to managers focused on alternative investments who manage hedge funds, funds of hedge funds, private equity funds and real estate funds, across registered, partnership and separate account structures domiciled in the United States and overseas. Revenues from investment operations platforms are recognized in Asset management, administration and distribution fees on the accompanying Consolidated Statements of Operations.

Principles of Consolidation

The Consolidated Financial Statements include the accounts of the Company and its wholly-owned subsidiaries and entities in which it holds a controlling financial interest. The Company determines whether it has a controlling financial interest either by its decision-making ability through voting interests or by the extent of the Company’s participation in the economic risks and rewards of the entity through variable interests. The Company’s principal subsidiaries are SEI Investments Distribution Co. (SIDCO), SEI Investments Management Corporation (SIMC), SEI Private Trust Company (SPTC), SEI Trust Company (STC), SEI Global Services, Inc. (SGSI) and SEI Investments (Europe) Limited (SIEL). All intercompany accounts and transactions have been eliminated.

The Company accounts for investments in unconsolidated entities that are 20 percent to 50 percent owned or are 20 percent or less owned and have the ability to exercise significant influence over the operating and financial policies of the entity under the equity method of accounting. Under this method of accounting, the Company’s interest in the net assets of unconsolidated entities is reflected in Investment in unconsolidated affiliates on the accompanying Consolidated Balance Sheet and its interest in the earnings or losses of unconsolidated entities is reflected in Equity in earnings of unconsolidated affiliates on the accompanying Consolidated Statement of Operations.

Variable Interest Entities

The Company has involvement with various variable interest entities (VIE or VIEs) primarily co-sponsored investment products established for clients created in the form of various types of legal entity structures. Effective January 1, 2016, the Company adopted the amendments contained in Accounting Standards Update (ASU) No. 2015-02, Consolidation (Topic 810): Amendments to the Consolidation Analysis (ASU 2015-02) which amends the current guidance for both the VIE and the voting interest entity (VOE) consolidation models. This guidance rescinds the indefinite deferral of the VIE guidance for investment companies that permitted application of the risks and rewards based approach. The adoption of ASU 2015-02 did not have any affect on the consolidated financial statements and related disclosures (See Note 3).

Management’s Use of Estimates

The preparation of financial statements in conformity with accounting principles generally accepted in the United States (U.S. GAAP) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

Revenue Recognition

The Company’s principal sources of revenues are: (1) asset management, administration and distribution fees earned based upon a contractual percentage of net assets under management or administration; (2) information processing and software servicing fees that are either recurring and primarily earned based upon the number of trust accounts being serviced or a percentage of the total average daily market value of the clients' assets processed on the Company's platforms, or non-recurring and based upon project-oriented contractual agreements related to client implementations; and (3) transaction-based fees for providing trade-execution services.

The Company’s revenues are based on contractual arrangements. Revenues are recognized in the periods in which the related services are performed provided that persuasive evidence of an agreement exists, the fee is fixed or determinable, and collectibility is reasonably assured. Cash received by the Company in advance of the performance of services is deferred and recognized as revenue when earned. Reimbursements received for out-of-pocket expenses incurred are recorded as revenue. Certain portions of the Company’s revenues require management’s consideration of the nature of the client relationship in determining whether to recognize as revenue the gross amount billed or net amount retained after payments are made to suppliers for certain services related to the product or service offering. For the majority of the Company's services, the Company is the primary obligor responsible for fulfilling the performance obligations of the contract. In addition, management retains full discretion in establishing the price charged to the customer, control the nature, type, characteristics or specifications of the performance obligations identified in the contract, and assume all credit risk associated with the client. Based on the foregoing, fees received by the Company for these services are recorded as gross revenues and vendor costs are recorded as gross expenses. However, the Company is also party to certain arrangements whereby the Company is not the primary obligor responsible for fulfilling the performance obligations of the contract. Fees received for those arrangements are reported net of costs associated with the provision of those services.

Cash and Cash Equivalents

The Company considers investment instruments purchased with an original maturity of three months or less to be cash equivalents. Cash and cash equivalents include $401,292 and $374,760 at December 31, 2017 and 2016, respectively, primarily invested in SEI-sponsored open-ended money market mutual funds. The SEI-sponsored mutual funds are considered Level 1 assets.

Restricted Cash

Restricted cash includes $3,000 at December 31, 2017 and 2016 segregated for regulatory purposes related to trade-execution services conducted by SIEL. Restricted cash also includes $505 and $500 at December 31, 2017 and 2016, respectively, segregated in special reserve accounts for the benefit of SIDCO customers in accordance with certain rules established by the Securities and Exchange Commission for broker-dealers.

Allowances for Doubtful Accounts

The Company provides an allowance for doubtful accounts equal to the estimated uncollectible amounts. The Company’s estimate is based on historical collection experience and a review of the current status of trade accounts receivable.

Concentration of Credit Risk

Financial instruments which potentially expose the Company to concentrations of credit risk consist primarily of cash equivalents and trade receivables. Cash equivalents are principally invested in short-term money market funds or placed with major banks and high-credit qualified financial institutions. Cash deposits maintained with institutions are in excess of federally insured limits. Concentrations of credit risk with respect to the Company's receivables are limited due to the large number of clients and their dispersion across geographic areas. No single group or customer represents greater than ten percent of total accounts receivable.

Property and Equipment

Property and Equipment are recorded at cost. Expenditures for major additions and improvements are capitalized and minor replacements, maintenance, and repairs are charged to expense as incurred. Construction in progress includes the cost of construction and other direct costs attributable to the construction. When property and equipment are retired or disposed of, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations for the respective period. Depreciation is provided over the estimated useful lives using the straight line method for financial statement purposes. No provision for depreciation is made for construction in progress until such time as the relevant assets are completed and put into service. The Company uses other depreciation methods, generally accelerated, for tax purposes where appropriate. Buildings and building improvements are depreciated over 25 to 39 years. Equipment, purchased software and furniture and fixtures have useful lives ranging from 3 to 5 years. Amortization of leasehold improvements is computed using the straight line method over the shorter of the remaining lease term or the estimated useful lives of the improvements.

Marketable Securities

The classification of investments in marketable securities is determined at the time of purchase and reevaluated at each balance sheet date. Debt and equity securities classified as available-for-sale are reported at fair value as determined by the most recently traded price of each security at the balance sheet date. Unrealized gains and losses, net of income taxes, are reported as a separate component of comprehensive income. SIDCO, the Company’s broker-dealer subsidiary, reports changes in fair value of marketable securities through current period earnings due to specialized accounting practices related to investments by broker-dealers. The Company records its investments in funds sponsored by LSV on the accompanying Consolidated Balance Sheets at fair value. Unrealized gains and losses from the change in fair value of these securities are recognized in current period earnings. The specific identification method is used to compute the realized gains and losses on all of the Company’s marketable securities (See Note 6).

The Company evaluates the realizable value of its marketable securities on a quarterly basis. In the event that the carrying value of an investment exceeds its fair value and the decline in value is determined to be other-than-temporary, an impairment charge is recorded and a new cost basis for the investment is established. Some of the factors considered in determining other-than-temporary impairment for equity securities include, but are not limited to, significant or prolonged declines in the fair value of the investments, the Company’s ability and intent to retain the investment for a period sufficient to allow the value to recover, and the financial condition of the investment. Some of the factors considered in determining other-than-temporary impairment for debt securities include, but are not limited to, the intent of management to sell the security, the likelihood that the Company will be required to sell the security before recovering its cost, and management’s expectation to recover the entire amortized cost basis of the security even if there is no intent to sell the security.

Fair Value of Financial Instruments

Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The fair value hierarchy requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. The fair value hierarchy describes three levels of inputs that may be used by the Company to measure fair value:

Level 1 – Quoted prices in active markets for identical assets or liabilities without adjustment. The Company’s Level 1 assets primarily include investments in mutual funds sponsored by SEI that are quoted daily.

Level 2 – Observable inputs other than Level 1 prices, such as quoted prices for similar assets, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 2 financial assets consist of Government National Mortgage Association (GNMA) mortgage-backed securities, Federal Home Loan Bank (FHLB) and other U.S. government agency short-term notes. The investments in GNMA mortgage-backed securities were purchased for the sole purpose of satisfying applicable regulatory requirements imposed on our wholly-owned limited purpose federal thrift subsidiary, SPTC. The investments in FHLB and other U.S. government agency short-term notes were purchased as part of a cash management program requiring only short term, top-tier investment grade government and corporate securities.

Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. Level 3 assets and liabilities include financial instruments whose value is determined using pricing models, or similar techniques, as well as instruments for which the determination of fair value requires significant judgment by management. The Company had no Level 3 financial assets or liabilities at December 31, 2017 or 2016 that were required to be measured at fair value on a recurring basis.

The fair value of an asset or liability may include inputs from more than one level in the fair value hierarchy. The lowest level of significant inputs used to value the asset or liability determines which level the asset or liability is classified in its entirety. Transfers between levels of the fair value hierarchy are reported at fair value as of the beginning of the period in which the transfers occur.

See Note 5 for information on related disclosures regarding fair value measurements.

Capitalized Software

Costs incurred for the development of internal use software to be offered in a hosting arrangement is capitalized during the development stage of the software application. These costs include direct external and internal costs to design the software configuration and interfaces, coding, installation, and testing. Costs incurred during the preliminary and post-implementation stages of the software application are expensed as incurred. Costs associated with significant enhancements to a software application are capitalized while costs incurred to maintain existing software applications are expensed as incurred. The capitalization of software development costs requires considerable judgment by management with respect to certain external factors, including, but not limited to, technological and economic feasibility, and estimated economic life. Amortization of capitalized software development costs begins when the product is ready for its intended use. Capitalized software development

costs are amortized on a product-by-product basis using the straight-line method over the estimated economic life of the product or enhancement.

The Company capitalized $61,043, $50,392 and $29,416 of software development costs during 2017, 2016 and 2015, respectively. The Company's capitalized software development costs primarily relate to the further development of the SEI Wealth Platform^SM(the Platform). The Company capitalized $51,353, $39,785 and $24,515 of software development costs for significant enhancements to the Platform during 2017, 2016 and 2015, respectively. As of December 31, 2017, the net book value of the Platform was $285,207. The net book value includes $38,093 of capitalized software development costs in-progress associated with future releases.

The Company also capitalized $9,690, $10,607 and $4,901 of software development costs during 2017, 2016 and 2015, respectively, related to an application for the Investment Managers segment. Capitalized software development costs in-progress at December 31, 2017 associated with this application were $25,198. The application is expected to be placed into service during the first quarter 2018.

The amount of amortization expense recognized related to the SEI Wealth Platform is based upon management’s estimate of its useful life. Management continually reassesses the estimated useful life of the Platform and any change in management’s estimate could result in the remaining amortization expense to be accelerated or spread out over a longer period. The initial version of the Platform was placed into service in July 2007 with an estimated useful life of 15 years. All significant enhancements to the Platform were being amortized over the remaining useful life and would have been fully amortized by June 2022. As a result of recent development efforts, successful client installation onto the Platform of a large-scale regional bank and market acceptance as evidenced by a contractual arrangement with a large-scale national bank, the Company adjusted the remaining useful life of certain components and functionality of the Platform that were placed into service during the past several years. This change in estimate was applied prospectively effective October 1, 2017. The initial version of the Platform and certain subsequent releases were not adjusted. The adjustment resulted in a decrease to the Company's amortization expense in 2017 of $4,347, and accordingly, increased income from operations by $4,347 and increased net income by $3,156, or $0.02 diluted earnings per share. As of December 31, 2017, the Platform has a weighted average remaining life of 8.2 years. Amortization expense for the Platform was $46,505, $45,047 and $42,401 in 2017, 2016 and 2015, respectively, and is included in Amortization expense on the accompanying Consolidated Statements of Operations.

The Company currently expects to recognize amortization expense related to all capitalized software development costs placed into service as of December 31, 2017 each year from 2018 through 2022 as follows:



2018	$	43,717
2019	43,717
2020	43,717
2021	43,439
2022	29,556

The Company evaluates the carrying value of capitalized software development costs when circumstances indicate the carrying value may not be recoverable. The review of capitalized software development costs for impairment requires significant assumptions about operating strategies, underlying technologies utilized, and external market factors. External market factors include, but are not limited to, expected levels of competition, barriers to entry by potential competitors, stability in the target market and governmental regulations. During 2015, the Company determined that specific functionality within the Platform was no longer in use and wrote off $5,533 of previously capitalized software development costs reported under the Private Banks and Investment Advisors business segments. The expense associated with the write off is included in Facilities, supplies and other costs on the accompanying Consolidated Statement of Operations. The Company did not recognize any impairment charges related to its capitalized software development costs in 2017 or 2016.

Business Combinations

The Company accounts for business combinations in accordance with Accounting Standards Codification Topic 805, Business Combinations (ASC 805). ASC 805 establishes principles and requirements for recognizing the total consideration transferred, assets acquired and liabilities assumed in a business combination. ASC 805 also provides guidance for recognizing and measuring goodwill acquired in a business combination and requires the acquirer to disclose information needed to evaluate and understand the financial impact of the business combination. The Company recognizes assets and liabilities acquired at their estimated fair values. Management uses judgment to identify the acquired assets and liabilities assumed; estimate the fair value of these assets and liabilities; estimate the useful life of the assets; and assess the appropriate method for recognizing depreciation or amortization expense over the estimated useful life of the assets.

Goodwill and Other Intangible Assets

The Company reviews long-lived assets and identifiable definite-lived intangible assets for impairment at least annually or whenever events or changes in circumstances indicate that the carrying amount of these assets may not be recoverable. For purposes of recognizing and measuring an impairment loss, a long-lived asset is grouped with other assets and liabilities at the lowest level for which identifiable cash flows are largely independent.

Identifiable definite-lived intangible assets on the Company’s Consolidated Balance Sheet are amortized on a straight-line basis according to their estimated useful lives. Goodwill is not amortized but is reviewed for impairment annually or more frequently if an event occurs or circumstances change that would more likely than not reduce the fair value of a reporting unit below its carrying amount. Current guidance require that a two-step, fair value based test be performed to assess goodwill for impairment. In the first step, the fair value of each reporting unit is compared with its carrying value, including goodwill. If the fair value exceeds the carrying value, goodwill is not impaired and no further testing is performed. The second step is performed if the carrying value exceeds the fair value. The second step requires an allocation of fair value to the individual assets and liabilities using a purchase price allocation in order to determine the implied fair value of goodwill. If the implied fair value of goodwill is less than the carrying amount, an impairment loss is recognized.

Income Taxes

The Company applies the asset and liability approach to account for income taxes whereby deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled.

Foreign Currency Translation

The assets and liabilities and results of operations of the Company’s foreign subsidiaries are measured using the foreign subsidiary’s local currency as the functional currency. Assets and liabilities have been translated into U.S. dollars using the rates of exchange at the balance sheet dates. The results of operations have been translated into U.S. dollars at average exchange rates prevailing during the period. The resulting translation gain and loss adjustments are recorded as a separate component of comprehensive income.

Transaction gains and losses from exchange rate fluctuations are included in the results of operations in the periods in which they occur. There were no material gains or losses from exchange rate fluctuations in 2017, 2016 or 2015.

Earnings Per Common Share

Basic earnings per common share is computed by dividing net income attributable to SEI Investments common shareholders by the weighted average number of common shares outstanding during the period. Diluted earnings per common share is computed by dividing net income attributable to SEI Investments common shareholders by the combination of the weighted average number of common shares outstanding and the dilutive potential common shares, such as stock options, outstanding during the period.

The calculations of basic and diluted earnings per share for 2017, 2016 and 2015 are:



	2017		2016		2015
Net income	$	404,389	$	333,817	$	331,655
Shares used to compute basic earnings per common share	158,177,000		161,350,000		165,725,000
Dilutive effect of stock options	4,092,000		3,081,000		3,873,000
Shares used to compute diluted earnings per common share	162,269,000		164,431,000		169,598,000
Basic earnings per common share	$	2.56	$	2.07	$	2.00
Diluted earnings per common share	$	2.49	$	2.03	$	1.96

Employee stock options to purchase approximately 5,196,000, 10,632,000 and 10,730,000 shares of common stock, with an average exercise price per share of $45.49, $35.02 and $33.99, were outstanding during 2017, 2016 and 2015, respectively, but not included in the computation of diluted earnings per common share because either the performance conditions have not been satisfied or would have been satisfied if the reporting date was the end of the contingency period or the option’s exercise price was greater than the average market price of the Company’s common stock and the effect on diluted earnings per common share would have been anti-dilutive (See Note 8).

Stock-Based Compensation

Stock-based compensation cost is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which is the vesting period. The amount of stock-based compensation expense that is recognized in a given period is dependent upon management’s estimate of when the vesting targets are expected to be achieved. If this estimate proves to be inaccurate, the remaining amount of stock-based compensation expense could be accelerated, spread out over a longer period, or reversed (See Note 8).

The Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASU 2016-09) during 2017. As required by ASU 2016-09, excess tax benefits recognized on stock-based compensation expense are reflected in the accompanying Consolidated Statements of Operations as a component of the provision for income taxes on a prospective basis (See Note 12). Additionally, excess tax benefits or deficiencies recognized on stock-based compensation expense are classified as an operating activity in the accompanying Consolidated Statements of Cash Flows. The Company has applied this provision retrospectively for all periods presented. As a result, net cash provided by operating activities for 2016 and 2015 increased by $8,984 and $16,056, respectively, with corresponding offsets to net cash used for financing activities.

ASU 2016-09 also allows for the option to account for forfeitures as they occur when determining the amount of compensation cost to be recognized, rather than estimating expected forfeitures over the course of a vesting period. The Company elected to account for forfeitures as they occur. In addition, ASU 2016-09 eliminates anticipated windfalls and shortfalls that were included in the calculation of assumed proceeds for computing the dilutive effect of share-based payment awards in the calculation of diluted earnings per share. No adjustments to the Company's prior period reported diluted earnings per share amounts were permitted by ASU 2016-09.

The net cumulative effect to the Company from the adoption of ASU 2016-09 was an increase to paid-in capital of $2,582, a reduction to retained earnings of $1,669 and an increase to deferred tax assets of $913 as of January 1, 2017.

New Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (FASB) issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). ASU 2014-09 provides a single comprehensive model to be used in the accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance. The principal concept in ASU 2014-09 is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The new standard requires an entity to apply a five-step model that includes identifying the contract with a customer, identifying the performance obligations in the contract, determining the transaction price, allocating the transaction price to the performance obligations, and recognizing revenue when (or as) an entity satisfies a performance obligation. The standard also specifies the accounting for certain costs to obtain or fulfill a contract with a customer and requires expanded disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers.

The FASB issued several amendments to ASU 2014-09. These amendments include, but are not limited to, clarification of principal versus agent guidance in situations in which a revenue transaction involves a third party in providing goods or services to a customer. In such circumstances, an entity must determine whether the nature of its promise to the customer is to provide the underlying goods or services (the entity is the principal in the transaction and reports the transaction gross) or to arrange for a third party to provide the underlying goods or services (the entity is the agent in the transaction and reports the transaction net). The amendments also clarify, in terms of identifying performance obligations, how entities would determine whether promised goods or services are separately identifiable from other promises in a contract and, therefore, would be accounted for separately.

ASU 2014-09 is effective for the Company on January 1, 2018 and provides for either full retrospective adoption or a modified retrospective adoption by which it is applied only to the most current period presented. The Company has concluded that it will utilize the modified retrospective method of adoption with a cumulative catch up adjustment and will provide additional disclosures in its 2018 consolidated financial statements. The Company has completed analyses, executed project management relative to the process of adopting ASU 2014-09, and conducted detailed reviews of customer contracts to complete necessary adjustments to existing accounting policies. The Company has implemented changes to its accounting systems, processes and internal controls over revenue reporting and the new disclosure requirements, as necessary, in the application of the new standard.

The adoption of ASU 2014-09 did not change the accounting for the majority of the Company’s revenue arrangements and will not have a material impact to the Company’s consolidated financial statements. A summary of the Company’s assessment and the primary changes that will be implemented in the first quarter of 2018 due to the adoption of the new standard are presented below:

•The majority of the Company’s services are bundled together, and provided and completed for the client on a monthly basis. For these revenue arrangements, the Company will continue to recognize revenue on a monthly basis as the client

consumes the benefits continuously over time. The timing and recognition of revenues from these arrangements will not change as a result of the adoption of ASU 2014-09.

•Some contracts include implementation fees, which are recognized in Information processing and software servicing fees, and fund conversion fees, which are recognized in Asset management, administration and distribution fees. The Company concluded that for most of the arrangements that included implementation or fund conversion services were a distinct and separate performance obligation. As a result, upon the adoption of ASU 2014-09, the timing and recognition of fees from most of these arrangements did not change. However, future revenue arrangements that include implementation or fund conversion services may not be considered distinct performance obligations which would require such fees to be recognized over the life of the client which could longer. The Company will evaluate each contract in accordance with the requirements of ASU 2014-09.

•Research services provided by SIDCO, the Company’s broker-dealer subsidiary, to customers in soft-dollar arrangements were determined to be a separate performance obligation that should be allocated a portion of the transaction price. Research services provided by a broker-dealer may be internally generated or provided by a third party and paid directly by the broker-dealer on the customer’s behalf. It was determined that SIDCO is considered an agent since it does not control the research services before they are transferred to the customer. Therefore, fees received for research services should be recorded net of amounts paid for the soft dollar arrangement. These amounts paid by the Company were previously recorded as an expense and beginning January 1, 2018 will be recorded net of any revenue recognized. The amounts recorded as an expense in 2017, 2016 and 2015 were $14,623, $18,409, and $20,333 respectively.

•Incremental contract acquisition costs, primarily sales compensation costs paid to the Company's sales personnel, related to information processing contracts in the Private Banks segment and fund administration contracts in the Investment Managers segment will be deferred and recognized over the expected client life. The deferred asset from contract acquisition costs is assessed for impairment on a periodic basis. The Company will utilize the practical expedient permitting the expensing of costs to obtain a contract when the expected amortization period is one year or less. As a result, incremental contract acquisition costs will be capitalized and subsequently amortized upon adoption on January 1, 2018 as a cumulative-effect adjustment to equity.

The Company is in the process of finalizing its assessment of the cumulative-effect adjustment of initially applying the new revenue standard but does not believe there are any remaining significant implementation items associated with the adoption of the new standard that have not yet been addressed. The cumulative-effect adjustment to equity of initially applying the new standard on January 1, 2018 is expected to be immaterial.

In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASU 2016-01) that will significantly change the income statement impact of equity investments held by an entity, and the recognition of changes in fair value of financial liabilities when the fair value option is elected. The Company adopted ASU 2016-01 during the first quarter 2018. Management currently believes the most significant impact will be the requirement to recognize all changes in fair value of available-for-sale equity securities in current period earnings. Previously, these changes in fair value were recognized as a separate component of comprehensive income. ASU 2016-01 will require a cumulative-effect adjustment to retained earnings as of January 1, 2018. The Company does not believe the cumulative-effect adjustment will have a material impact to its consolidated financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (ASU 2016-02) requiring lessees to recognize a right-of-use asset and a lease liability on the balance sheet for all leases with the exception of short-term leases. For lessees, leases will continue to be classified as either operating or finance leases in the income statement. Lessor accounting is similar to the current model but updated to align with certain changes to the lessee model. Lessors will continue to classify leases as operating, direct financing or sales-type leases. The new standard must be adopted using a modified retrospective transition and requires application of the new guidance at the beginning of the earliest comparative period presented. The updated standard is effective for the Company beginning in the first quarter of 2019. Early adoption is permitted. The Company is currently evaluating the transition method that will be elected and the effect that the updated standard will have on its consolidated financial statements and related disclosures.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (ASU 2016-13) which requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. ASU No. 2016-13 limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. ASU 2016-13 becomes effective for the Company during the first quarter of 2020. Early adoption is permitted. The Company is currently evaluating the effect that the updated standard will have on its consolidated financial statements and related disclosures.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (ASU 2016-18) which requires restricted cash and cash equivalents to be included with cash and cash equivalents when reconciling

beginning-of-period and end-of-period amounts shown on the statement of cash flows. Under this guidance, the statement of cash flows should explain the total change in cash balances, including amounts described as restricted. The Company adopted ASU 2016-18 during the first quarter 2018. The Company has determined ASU 2016-18 will not have a material impact on its consolidated financial statements and related disclosures.

In January 2017, the FASB issued ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASU 2017-04). The objective of ASU 2017-04 is to simplify the subsequent measurement of goodwill by entities performing their annual goodwill impairment tests by comparing the fair value of a reporting unit, including income tax effects from any tax-deductible goodwill, with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds fair value. ASU 2017-04 is effective for the Company beginning in the first quarter of 2020. Early adoption is permitted. The Company is currently evaluating the impact of adopting ASU 2017-04 on its consolidated financial statements and related disclosures.

Reclassifications

Certain prior year amounts have been reclassified to conform to current year presentation.

Investment In Unconsolidated Affiliates

12 Months Ended

Dec. 31, 2017

Equity Method Investments and Joint Ventures [Abstract]

Investment In Unconsolidated Affiliates

Investment in Unconsolidated Affiliates

LSV Asset Management

The Company has an investment in the general partnership LSV Asset Management (LSV), a registered investment advisor that provides investment advisory services primarily to institutions, including pension plans and investment companies. LSV is currently an investment sub-advisor for a limited number of SEI-sponsored mutual funds. As of December 31, 2017, the Company's total partnership interest in LSV was approximately 38.9 percent. The Company accounts for its interest in LSV using the equity method because of its less than 50 percent ownership. The Company’s interest in the net assets of LSV is reflected in Investment in unconsolidated affiliates on the accompanying Consolidated Balance Sheets and its interest in the earnings of LSV is reflected in Equity in earnings of unconsolidated affiliates on the accompanying Consolidated Statements of Operations.

At December 31, 2017, the Company’s total investment in LSV was $59,492. The Company’s proportionate share in the earnings of LSV was $152,550, $126,103 and $138,407 in 2017, 2016 and 2015, respectively. The Company receives partnership distributions related to the earnings of LSV on a quarterly basis. As such, the Company considers these distribution payments as returns on investment rather than returns of the Company's original investment in LSV and has therefore classified the associated cash inflows as an operating activity on the Consolidated Statements of Cash Flows. The Company received partnership distribution payments from LSV of $143,517, $125,224 and $141,767 in 2017, 2016 and 2015, respectively.

These tables contain condensed financial information of LSV:



Condensed Statement of Operations Year ended December 31,	2017		2016		2015
Revenues	$	491,872	$	399,462	$	427,653
Net income	$	392,141	$	323,381	$	352,845



Condensed Balance Sheets December 31,	2017		2016
Current assets	$	155,239	$	125,872
Non-current assets	1,407		1,927
Total assets	$	156,646	$	127,799

Current liabilities	$	46,486	$	39,303
Partners’ capital	110,160		88,496
Total liabilities and partners’ capital	$	156,646	$	127,799

Guaranty Agreement with LSV Employee Group III

In October 2012, a group of existing employees of LSV formed a new limited liability company called LSV Employee Group III and agreed to purchase a portion of the partnership interest of existing LSV employees for $77,700, of which $69,930 was financed through syndicated term loan facilities contained in a credit agreement with CIBC Bank USA (formerly The PrivateBank and Trust Company). LSV Employee Group III owns the purchased partnership interest. The Company provided an unsecured guaranty for $45,000 of the obligations of LSV Employee Group III to the lenders through a guaranty agreement.

In addition, LSV agreed to provide an unsecured guaranty for the remaining $24,930 of the obligations of LSV Employee Group III to the lenders through a separate guaranty agreement.

The Company’s direct interest in LSV was unchanged as a result of this transaction. The Company determined that LSV Employee Group III was a VIE; however, the Company was not considered the primary beneficiary because it did not have the power to direct the activities that most significantly impact the economic performance of LSV Employee Group III either directly or through any financial responsibility from the guaranty.

In September 2014 and June 2017, LSV Employee Group III made the final principal payments related to the term loans guaranteed by LSV and the Company, respectively, and has no further obligation regarding the agreement. The Company has no other interests in LSV Employee Group III and, therefore, no longer considers LSV Employee Group III to be a VIE.

Variable Interest Entities - Investment Products	12 Months Ended
Variable Interest Entities - Investment Products	Dec. 31, 2017
Variable Interest Entities Disclosure [Abstract]
Variable Interest Entities - Investment Products	Variable Interest Entities – Investment Products The Company or its affiliates have created numerous investment products for its clients in various types of legal entity structures. The Company serves as the Manager, Administrator and Distributor for these investment products and may also serve as the Trustee for some of the investment products. The Company receives asset management, distribution, administration and custodial fees for these services. Clients are the equity investors and participate in proportion to their ownership percentage in the net income or loss and net capital gains or losses of the products, and, on liquidation, will participate in proportion to their ownership percentage in the remaining net assets of the products after satisfaction of outstanding liabilities. The Company has concluded that it is not the primary beneficiary of the entities and; therefore, is not required to consolidate any of the pooled investment vehicles for which it receives asset management, distribution, administration and custodial fees under the VIE model. The entities either do not meet the definition of a VIE or the Company does not hold a variable interest in the entities. The entities either qualify for the money market scope exception, or are entities in which the Company’s asset management, distribution, administration and custodial fees are commensurate with the services provided and include fair terms and conditions, or are entities that are limited partnerships which have substantive kick-out rights. The Company acts as a fiduciary and does not hold any other interests other than insignificant seed money investments in the pooled investment vehicles. For this reason, the Company also concluded that it is not required to consolidate the pooled investment vehicles under the voting interest entity (VOE) model. The Company is a party to expense limitation agreements with certain SEI-sponsored money market funds subject to Rule 2a-7 of the Investment Company Act of 1940 which establish a maximum level of ordinary operating expenses incurred by the fund in any fiscal year including, but not limited to, fees of the administrator or its affiliates. Under the terms of these agreements, the Company waived $27,434, $41,227 and $55,713 in fees during 2017, 2016 and 2015, respectively.

Composition of Certain Financial Statement Captions

12 Months Ended

Dec. 31, 2017

Items Included in Consolidated Statement of Financial Condition [Abstract]

Composition of Certain Financial Statement Captions

Receivables

Receivables on the accompanying Consolidated Balance Sheets consist of:



	2017			2016
Trade receivables	$	76,760		$	48,683
Fees earned, not billed	194,331			168,971
Other receivables	12,310			10,826
	283,401			228,480
Less: Allowance for doubtful accounts	(695		)	(523		)
Receivables, net	$	282,706		$	227,957

Fees earned, not billed represents receivables earned but unbilled and results from timing differences between services provided and contractual billing schedules. These billing schedules generally provide for fees to be billed on a quarterly basis. In addition, certain fees earned from investment operations services are calculated based on assets under administration that have a prolonged valuation process which delays billings to clients.

Property and Equipment

Property and Equipment on the accompanying Consolidated Balance Sheets consists of:



	2017			2016
Buildings	$	153,961		$	152,171
Equipment	115,546			106,759
Land	10,030			10,030
Purchased software	134,610			128,008
Furniture and fixtures	18,114			17,292
Leasehold improvements	18,017			15,175
Construction in progress	6,105			2,077
	456,383			431,512
Less: Accumulated depreciation	(309,955		)	(285,322		)
Property and Equipment, net	$	146,428		$	146,190

Depreciation expense related to property and equipment for 2017, 2016 and 2015 was $27,311, $26,440 and $24,044, respectively.

Other Assets

Other assets consist of long-term prepaid expenses, deposits, other investments at cost and various other assets. Amortization expense for certain other assets for 2017, 2016 and 2015 was $218, $345 and $229, respectively.

Accrued Liabilities

Accrued Liabilities on the accompanying Consolidated Balance Sheets consist of:



	2017		2016
Accrued employee compensation	$	88,960	$	79,735
Accrued consulting, outsourcing and professional fees	29,658		24,428
Accrued sub-advisory, distribution and other asset management fees	42,365		41,666
Accrued dividend payable	47,179		44,596
Other accrued liabilities	56,896		50,100
Accrued liabilities	$	265,058	$	240,525

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

The fair value of the Company’s financial assets and liabilities, except for the Company's investment funds sponsored by LSV, is determined in accordance with the fair value hierarchy. The fair value of the Company’s Level 1 financial assets consists mainly of investments in open-ended mutual funds that are quoted daily. Level 2 financial assets consist of Government National Mortgage Association (GNMA) mortgage-backed securities held by the Company's wholly-owned limited purpose federal thrift subsidiary, SEI Private Trust Company (SPTC), Federal Home Loan Bank (FHLB) and other U.S. government agency short-term notes held by SIDCO. The financial assets held by SIDCO were purchased as part of a cash management program requiring only short term, top-tier investment grade government and corporate securities. The financial assets held by SPTC are debt securities issued by GNMA and are backed by the full faith and credit of the U.S. government. These securities were purchased for the sole purpose of satisfying applicable regulatory requirements and have maturity dates which range from 2020 to 2041.

The fair value of the Company's investment funds sponsored by LSV is measured using the net asset value per share (NAV) as a practical expedient. The NAVs of the funds are calculated by the funds' independent custodian and are derived from the fair values of the underlying investments as of the reporting date. The funds allow for investor redemptions at the end of each calendar month. This investment has not been classified in the fair value hierarchy but is presented in the tables below to permit reconciliation to the amounts presented on the accompanying Consolidated Balance Sheets.

The valuation of the Company's Level 2 financial assets held by SIDCO and SPTC are based upon securities pricing policies and procedures utilized by third-party pricing vendors. The Company had no Level 3 financial assets or liabilities at December 31, 2017 or 2016 that were required to be measured at fair value on a recurring basis. There were no transfers of financial assets between levels within the fair value hierarchy during 2017.

Valuation of GNMA and Other U.S. Government Agency Securities

All of the Company's investments in GNMA, FHLB and other U.S. government agency securities are held in accounts at well-established financial institutions. The Company's selection of a financial institution for the purpose of purchasing securities considered a number of various factors including, but not limited to, securities pricing policies and procedures utilized by that financial institution. Each financial institution utilizes the services of independent pricing vendors. These vendors utilize evaluated and industry accepted pricing models that vary by asset class and incorporate available trade, bid and other market information to determine the fair value of the securities. The market inputs, listed in approximate order of priority, include: benchmark yields, reported trade, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data. The Company evaluated the information regarding the pricing methodologies and processes utilized by the independent pricing vendors during the selection process of the financial institution. The Company analyzed this information for the purpose of classifying the securities into the appropriate level within the fair value hierarchy and to ensure that each pricing model for each asset class provided the fair value of those specific securities in accordance with generally accepted accounting principles. The Company continually monitors the price of each security for any unanticipated deviations from the previously quoted price or deviations from anticipated changes in a security's price based upon an assessment of market factors and other factors relative to a specific issue expected to affect a security's price. In the event of any unanticipated deviations in a security's price, additional analysis is conducted which may include the comparison of the security's price as determined by other independent pricing vendors. The Company's investments in GNMA, FHLB and other U.S. government agency securities have been recorded at the prices provided by the independent pricing vendor without adjustment.

The fair value of certain financial assets and liabilities of the Company was determined using the following inputs:



			Fair Value Measurements at Reporting Date Using
	December 31, 2017		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
Assets
Equity available-for-sale securities	$	11,250	$	11,250	$	—
Fixed-income available-for-sale securities	76,733		—		76,733
Fixed-income securities owned	21,526		—		21,526
Investment funds sponsored by LSV (1)	6,034
	$	115,543	$	11,250	$	98,259



			Fair Value Measurements at Reporting Date Using
	December 31, 2016		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
Assets
Equity available-for-sale securities	$	9,581	$	9,581	$	—
Fixed-income available-for-sale securities	74,452		—		74,452
Fixed-income securities owned	21,339		—		21,339
Investment funds sponsored by LSV (1)	4,858
	$	110,230	$	9,581	$	95,791

(1) The fair value amounts presented in the tables above are intended to permit reconciliation of the fair value hierarchy to the amounts presented on the accompanying Consolidated Balance Sheets (See Note 6).

Marketable Securities

12 Months Ended

Dec. 31, 2017

Marketable Securities [Abstract]

Marketable Securities

Investments Available For Sale

Investments available for sale classified as non-current assets consist of:



	At December 31, 2017
	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)			Fair Value
SEI-sponsored mutual funds	$	7,369	$	110	$	(143	)	$	7,336
Equities and other mutual funds	3,456		458		—			3,914
Debt securities	77,745		—		(1,012		)	76,733
	$	88,570	$	568	$	(1,155	)	$	87,983



	At December 31, 2016
	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)			Fair Value
SEI-sponsored mutual funds	$	7,357	$	24	$	(996	)	$	6,385
Equities and other mutual funds	2,968		228		—			3,196
Debt securities	74,843		—		(391		)	74,452
	$	85,168	$	252	$	(1,387	)	$	84,033

Net unrealized holding losses at December 31, 2017 and 2016 were $386 (net of income tax benefit of $201) and $836 (net of income tax benefit of $299), respectively. These net unrealized losses are reported as a separate component of Accumulated other comprehensive loss on the accompanying Consolidated Balance Sheets.

There were gross realized gains of $529 and gross realized losses of $888 from available-for-sale securities during 2017. In 2016, there were gross realized gains of $284 and gross realized losses of $869 from available-for-sale securities. There were gross realized gains of $489 and gross realized losses of $729 from available-for-sale securities during 2015. Gains and losses from available-for-sale securities, including amounts reclassified from accumulated comprehensive income (loss), are reflected in Net gain (loss) from investments on the accompanying Consolidated Statements of Operations.

Investments in Affiliated Funds

The Company has an investment related to the startup of investment funds sponsored by LSV. The Company records this investment on the accompanying Consolidated Balance Sheets at fair value. Unrealized gains and losses from the change in fair value of these funds are recognized in Net gain (loss) from investments on the accompanying Consolidated Statements of Operations.

The investment primarily consists of U.S. dollar denominated funds that invest in equity securities of Canadian, Australian and Japanese companies. The underlying securities held by the funds are translated into U.S. dollars within the funds. The funds had a fair value of $6,034 and $4,858 at December 31, 2017 and 2016, respectively. The Company recognized gains of $1,176 and $819 and losses of $389 from the change in fair value of the funds during 2017, 2016 and 2015, respectively.

Securities Owned

The Company’s broker-dealer subsidiary, SIDCO, has investments in U.S. government agency securities with maturity dates less than one year. These investments are reflected as Securities owned on the accompanying Consolidated Balance Sheets. Due to specialized accounting practices applicable to investments by broker-dealers, the securities are reported at fair value and changes in fair value are recorded in current period earnings. The securities had a fair value of $21,526 and $21,339 at December 31, 2017 and 2016, respectively. There were no material net gains or losses from the change in fair value of the securities during 2017, 2016 and 2015.

Line Of Credit	12 Months Ended
Line Of Credit	Dec. 31, 2017
Line of Credit Facility [Abstract]
Lines Of Credit	Line of Credit On June 13, 2016, the Company entered into a five-year $300,000 Credit Agreement (the Credit Facility) with Wells Fargo Bank, National Association (Wells Fargo) and a syndicate of other lenders. The Credit Facility is scheduled to expire in June 2021, at which time any aggregate principal amount of loans outstanding becomes payable in full. Any borrowings made under the Credit Facility will accrue interest at rates that, at the Company's option, are based on a base rate (the Base Rate) plus a premium that can range from 0.25 percent to 1.00 percent or the London InterBank Offered Rate (LIBOR) plus a premium that can range from 1.25 percent to 2.00 percent depending on the Company’s Leverage Ratio (a ratio of consolidated indebtedness to consolidated EBITDA for the four preceding fiscal quarters, all as defined in the related agreement). The Base Rate is defined as the highest of a) the Federal Funds Rate, as published by the Federal Reserve Bank of New York, plus 0.50 percent, b) the prime commercial lending rate of Wells Fargo, c) the applicable LIBOR plus 1.00 percent, or d) 0 percent. The Company also pays quarterly commitment fees based on the unused portion of the Credit Facility. The quarterly fees for the Credit Facility can range from 0.15 percent of the amount of the unused portion to 0.30 percent, depending on the Company’s Leverage Ratio. Certain wholly-owned subsidiaries of the Company have guaranteed the obligations of the Company under the agreement. The aggregate amount of the Credit Facility may be increased by an additional $100,000 under certain conditions set forth in the agreement. The Credit Facility contains covenants that restrict the ability of the Company to engage in mergers, consolidations, asset sales, investments, transactions with affiliates, or to incur liens, as defined in the agreement. In the event of a default under the Credit Facility, the Company would also be restricted from paying dividends on, or repurchasing, its common stock without the approval of the lenders. None of the covenants of the Credit Facility negatively affect the Company’s liquidity or capital resources. Upon the occurrence of certain financial or economic events, significant corporate events, or certain other events of default constituting an event of default under the Credit Facility, all loans outstanding may be declared immediately due and payable and all commitments under the agreement may be terminated. In July 2017, the Company borrowed $40,000 under the Credit Facility for the funding of an acquisition (See Note 16). In October 2017, the Company made a principal payment of $10,000. As of December 31, 2017, the outstanding balance of the Credit Facility was $30,000 and is included in Borrowings Under Revolving Credit Facility on the accompanying Consolidated Balance Sheet. The Company was in compliance with all covenants of the Credit Facility during 2017. In January 2018, the Company made a principal payment of $10,000. As of January 31, 2018, the amount of the Credit Facility that is available for general corporate purposes was $280,000. The Company considers the book value of long-term debt related to the borrowings through the Credit Facility to be representative of its fair value. The Company incurred $781, $531 and $483 in interest charges and commitment fees relating to the current and since expired lines of credit during 2017, 2016 and 2015, respectively, which are reflected in Interest expense on the accompanying Consolidated Statements of Operations. The weighted average interest rate applied to the outstanding balance of the Credit Facility during 2017 was 2.58 percent.

Shareholders' Equity

12 Months Ended

Dec. 31, 2017

Stockholders' Equity Note [Abstract]

Shareholders' Equity

Shareholders’ Equity

Stock-Based Compensation

The Company's active equity compensation plan, the 2014 Omnibus Equity Compensation Plan (the 2014 Plan), is the successor plan to the 2007 Equity Compensation Plan (the 2007 Plan) which was merged with and into the 2014 Plan in May 2014. The 2014 Plan provides for the grant of stock options, stock units, stock awards, stock appreciation rights, dividend equivalents and other stock-based awards.

Outstanding grants under the 2007 Plan will continue according to the terms in effect before the plan merger, but the outstanding shares will be issued or transferred under the 2014 Plan. Permitted grantees under the 2014 Plan include employees, non-employee directors and consultants who perform services for the Company. The plan is administered by the Compensation Committee of the Board of Directors of the Company. The Company has only non-qualified stock options outstanding under the 2014 Plan.

All outstanding stock options have performance-based vesting provisions that tie the vesting of stock options to the Company’s financial performance. The Company’s stock options vest at a rate of 50 percent when a specified diluted earnings per share target is achieved, and the remaining 50 percent when a second, higher-specified diluted earnings per share target is achieved. Options do not vest due to the passage of time but as a result of achievement of the financial vesting targets. Options granted in December 2017 include a service condition which requires a minimum two or four year waiting period from the grant date along with the attainment of the applicable financial vesting target. Earnings per share targets exclude the impact of stock-based compensation and are established at time of grant. The targets are measured annually on December 31. The amount of stock-based compensation expense recognized in the period is based upon management’s estimate of when the earnings per share targets may be achieved. Any change in management’s estimate could result in the remaining amount of stock-based compensation expense to be accelerated, spread out over a longer period, or reversed. This may cause volatility in the recognition of stock-based compensation expense in future periods and could materially affect the Company’s earnings.

The Company uses the Black-Scholes option pricing model to determine the fair value of stock options. The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the price of the Company’s common stock as well as other variables. These variables include expected stock price volatility over the term of the awards, actual and projected employee stock exercise behaviors, risk-free interest rate and expected dividends. The

Company primarily uses historical data to estimate the variables used in the option-pricing model except expected volatility. The Company uses a combination of historical and implied volatility.

The weighted average fair value of the Company’s stock options granted during 2017, 2016 and 2015 were $16.78, $12.43 and $12.16, respectively, using the following assumptions:



	2017		2016		2015
Expected term (in years)	6.00		6.00		5.58
Expected volatility	22.58	%	25.44	%	23.86	%
Expected dividend yield	0.82	%	1.10	%	1.00	%
Risk-free interest rate	2.29	%	2.18	%	1.90	%

The Company recognized stock-based compensation expense in its Consolidated Financial Statements in 2017, 2016 and 2015 as follows:



	2017			2016			2015
Stock-based compensation expense	$	36,366		$	16,017		$	17,312
Less: Deferred tax benefit	(7,891		)	(5,612		)	(6,107		)
Stock-based compensation expense, net of tax	$	28,475		$	10,405		$	11,205

During 2017, 2016 and 2015, the Company revised its estimates of when some vesting targets are expected to be achieved. The change in management’s estimate during 2017 resulted in an increase of $11,206 in stock-based compensation expense in 2017. The changes in management’s estimates during 2016 or 2015 were not material.

As of December 31, 2017, there was approximately 6,088,000 unvested employee stock options with an unrecognized compensation cost of $62,070 that the Company expects will vest and be expensed through 2022 with a weighted average period of 2.3 years.

This table presents certain information relating to the Company’s stock option plans for 2017, 2016 and 2015:



	Number of Shares		Weighted Avg. Price
Balance as of December 31, 2014	20,461,000		$	25.26
Granted	2,005,000		53.34
Exercised	(2,927,000	)	21.44
Expired or canceled	(302,000	)	28.97
Balance as of December 31, 2015	19,237,000		$	28.71
Granted	2,310,000		49.57
Exercised	(1,809,000	)	24.82
Expired or canceled	(1,669,000	)	30.86
Balance as of December 31, 2016	18,069,000		$	31.57
Granted	2,057,000		69.87
Exercised	(2,370,000	)	21.22
Expired or canceled	(1,044,000	)	33.42
Balance as of December 31, 2017	16,712,000		$	37.63

Exercisable as of December 31, 2017	10,624,000		$	28.99
Available for future grant as of December 31, 2017	24,108,000

As of December 31, 2016 and 2015, there were 6,692,000 and 8,508,000 shares exercisable, respectively. The expiration dates for options outstanding at December 31, 2017 range from December 16, 2018 to December 12, 2027 with a weighted average remaining contractual life of 6.3 years.

Upon exercise of stock options, the Company will issue new shares of its common shares. The Company does not hold any shares in treasury. The total intrinsic value of options exercised during 2017 and 2016 was $83,628 and $41,607, respectively. The total options exercisable as of December 31, 2017 had an intrinsic value of $455,528. The total options outstanding as of December 31, 2017 had an intrinsic value of $572,024. The total intrinsic value for options outstanding and options exercisable is calculated as the difference between the market value of the Company’s common stock as of

December 31, 2017 and the exercise price of the shares. The market value of the Company’s common stock as of December 31, 2017 was $71.86 as reported by the Nasdaq Stock Market, LLC.

This table summarizes information relating to all options outstanding and exercisable at December 31, 2017:



				Options Outstanding at December 31, 2017				Options Exercisable at December 31, 2017
Range of Exercise Prices (Per Share)				Number of Shares	Weighted Average Exercise Price (Per Share)		Weighted Average Remaining Contractual Life (Years)	Number of Shares	Weighted Average Exercise Price (Per Share)		Weighted Average Remaining Contractual Life (Years)
$	14.62	-	15.77	2,178,000	$	15.45	3.16	2,178,000	$	15.45	3.16
17.65		-	21.05	1,174,000	17.67		2.01	1,174,000	17.67		2.01
22.45		-	23.86	3,432,000	23.19		3.95	3,422,000	23.19		3.96
27.03		-	39.15	1,815,000	33.65		6.09	747,000	33.58		6.09
40.64		-	53.16	4,190,000	45.49		8.54	2,095,000	45.49		8.08
53.34		-	71.12	3,923,000	62.01		9.05	1,008,000	53.39		8.14
				16,712,000				10,624,000

Employee Stock Purchase Plan

The Company has an employee stock purchase plan that provides for offerings of common stock to eligible employees at a price equal to 85 percent of the fair market value of the stock at the end of the stock purchase period, as defined. The Company has reserved 15,600,000 shares for issuance under this plan. At December 31, 2017, 11,962,000 cumulative shares have been issued. There were no material costs incurred by the Company related to the employee stock purchase plan in 2017, 2016 and 2015.

Common Stock Buyback

The Board of Directors, under multiple authorizations, has authorized the purchase of the Company’s common stock on the open market or through private transactions. As of December 31, 2017, the Company had approximately $170,638 of authorization remaining for the purchase of common stock. The following table provides the total number of shares repurchased and the related total costs in 2017, 2016 and 2015:



Year	Total Number of Shares Repurchased	Total Cost
2017	4,403,000	$	248,114
2016	6,600,000	294,374
2015	5,951,000	289,587

The Company immediately retires its common stock when purchased. Upon retirement, the Company reduces Capital in excess of par value for the average capital per share outstanding and the remainder is charged against Retained earnings. If the Company reduces its Retained earnings to zero, any subsequent purchases of common stock will be charged entirely to Capital in excess of par value.

Rights Agreement

In December 2008, the Company’s Board of Directors declared a dividend distribution pursuant to a Rights Agreement (the Rights Agreement) which became effective on January 6, 2009. The purpose of the Rights Agreement is to deter coercive or unfair takeover tactics and to prevent a person or group (an Acquiring Person) from acquiring control of the Company without offering a fair price to all shareholders. Under the Rights Agreement, all common shareholders receive one Right for each common share outstanding. Each Right entitles the registered holder to purchase from the Company a unit consisting of one twenty-thousandths of a share of Series A Junior Participating Preferred Shares, $0.05 par value per share, or a combination of securities and assets of equivalent value, at a purchase price of $150.00 per unit, subject to adjustment. The Rights will become exercisable and trade separately from the common stock ten days following a public announcement that an Acquiring Person has beneficial ownership of more than 20 percent of the outstanding common stock of the Company or the commencement of a tender or exchange offer that would result in an Acquiring Person owning 20 percent or more of the outstanding common stock of the Company. Upon exercise, holders, other than an Acquiring Person, will have the right to purchase the common stock of the Company equal to twice the value of the exercise price of the Rights. In lieu of requiring payment of the purchase price upon exercise of the Rights following certain events, the Company may permit the holders simply to surrender the Rights, in which event they will be entitled to receive common shares and other property, as the case may be, with a value of 50 percent of what could be purchased by payment of the full purchase price. The Rights, which do not

have voting rights, will expire on January 6, 2019, and may be redeemed by the Company any time until ten days following the announcement of an Acquiring Person at a price of $0.01 per Right.

Cash Dividends

On May 24, 2017, the Board of Directors declared a cash dividend of $0.28 per share on the Company’s common stock, which was paid on June 16, 2017, to shareholders of record on June 7, 2017. On December 12, 2017, the Board of Directors declared a cash dividend of $0.30 per share on the Company’s common stock, which was paid on January 8, 2018, to shareholders of record on December 27, 2017.

The cash dividends declared in 2017, 2016 and 2015 were $91,444, $86,657 and $82,478, respectively. The Board of Directors has indicated its intention to declare future cash dividends on a semiannual basis.

Accumulated Other Comprehensive Income (Loss)

12 Months Ended

Dec. 31, 2017

Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]

Accumulated Other Comprehensive Income (Loss)

Other comprehensive income (loss) consists of net income and other gains and losses affecting shareholders’ equity that are excluded from net income. Other comprehensive income (loss) includes unrealized gains and losses on available for sale securities and foreign currency translation adjustments. The Company presents other comprehensive income (loss) in its Consolidated Statements of Comprehensive Income. Components of Accumulated other comprehensive income (loss), net of tax, consisted of:



	Foreign Currency Translation Adjustments			Unrealized Holding Gains (Losses) on Investments			Accumulated Other Comprehensive Income (Loss)
Balance, January 1, 2015	$	(10,088	)	$	1,193		$	(8,895	)

Other comprehensive loss before reclassifications	(14,900		)	(1,659		)	(16,559		)
Amounts reclassified from accumulated other comprehensive loss	—			164			164
Net current-period other comprehensive loss	(14,900		)	(1,495		)	(16,395		)

Balance, December 31, 2015	$	(24,988	)	$	(302	)	$	(25,290	)

Other comprehensive loss before reclassifications	(12,131		)	(918		)	(13,049		)
Amounts reclassified from accumulated other comprehensive loss	—			384			384
Net current-period other comprehensive loss	(12,131		)	(534		)	(12,665		)

Balance, December 31, 2016	$	(37,119	)	$	(836	)	$	(37,955	)

Other comprehensive gain before reclassifications	17,597			190			17,787
Amounts reclassified from accumulated other comprehensive loss	—			260			260
Net current-period other comprehensive gain	17,597			450			18,047

Balance, December 31, 2017	$	(19,522	)	$	(386	)	$	(19,908	)

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

The Company leases software, facilities, and data processing equipment under non-cancelable operating leases, some which contain escalation clauses for increased taxes and operating expenses. The Company has entered into maintenance agreements primarily for its data processing equipment. Rent expense, primarily related to user licenses for software, was $32,619, $28,016 and $25,074 in 2017, 2016 and 2015, respectively.

The aggregate noncancellable minimum commitments at December 31, 2017 are:



2018	$	10,046
2019	9,427
2020	7,413
2021	7,082
2022 and thereafter	29,171
	$	63,139

In the ordinary course of business, the Company from time to time enters into contracts containing indemnification obligations of the Company. These obligations may require the Company to make payments to another party upon the occurrence of certain events including the failure by the Company to meet its performance obligations under the contract. These contractual indemnification provisions are often standard contractual terms of the nature customarily found in the type of contracts entered into by the Company. In many cases, there are no stated or notional amounts included in the indemnification provisions. There are no amounts reflected on the Consolidated Balance Sheets as of December 31, 2017 and 2016 related to these indemnifications.

In the normal course of business, the Company is party to various claims and legal proceedings.

SEI has been named in seven lawsuits filed in Louisiana courts; four of the cases also name SPTC as a defendant. The underlying allegations in all actions relate to the purported role of SPTC in providing back-office services to Stanford Trust Company. The complaints allege that SEI and SPTC participated in some manner in the sale of “certificates of deposit” issued by Stanford International Bank so as to be a “seller” of the certificates of deposit for purposes of primary liability under the Louisiana Securities Law or so as to be secondarily liable under that statute for sales of certificates of deposit made by Stanford Trust Company. Two of the actions also include claims for violations of the Louisiana Racketeering Act and possibly conspiracy, and a third also asserts claims of negligence, breach of contract, breach of fiduciary duty, violations of the uniform fiduciaries law, negligent misrepresentation, detrimental reliance, violations of the Louisiana Racketeering Act, and conspiracy.

The procedural status of the seven cases varies. The Lillie case, filed originally in the 19th Judicial District Court for the Parish of East Baton Rouge, was brought as a class action and is procedurally the most advanced of the cases. SEI and SPTC filed exceptions, which the Court granted in part, dismissing claims under the Louisiana Unfair Trade Practices Act and permitting the claims under the Louisiana Securities Law to go forward. On March 11, 2013, newly-added insurance carrier defendants removed the case to the United States District Court for the Middle District of Louisiana. On August 7, 2013, the Judicial Panel on Multidistrict Litigation transferred the matter to the Northern District of Texas where MDL 2099, In re: Stanford Entities Securities Litigation (“the Stanford MDL”), is pending. On September 22, 2015, the District Court on the motion of SEI and SPTC dismissed plaintiffs’ claims for primary liability under Section 714(A) of the Louisiana Securities Law, but declined to dismiss plaintiffs’ claims for secondary liability under Section 714(B) of the Louisiana Securities Law based on the allegations pled by plaintiffs. On November 4, 2015, the District Court granted SEI and SPTC's motion to dismiss plaintiffs' claims under Section 712(D) of the Louisiana Securities Law. Consequently, the only claims of plaintiffs still pending before the District Court in Lillie are plaintiffs' claims for secondary liability against SEI and SPTC under Section 714(B) of the Louisiana Securities Law. On May 2, 2016, the District Court certified the class as being "all persons for whom Stanford Trust Company purchased or renewed Stanford Investment Bank Limited certificates of deposit in Louisiana between January 1, 2007 and February 13, 2009". Notice of the pendency of the class action was mailed to potential class members on October 4, 2016.

On December 1, 2016, a group of plaintiffs who opted out of the Lillie class filed a complaint against SEI and SPTC in the United States District Court in the Middle District of Louisiana, alleging claims essentially the same as those in Lillie. In January 2017, the Judicial Panel on Multidistrict Litigation transferred the proceeding to the Northern District of Texas and the Stanford MDL. During February 2017, SEI filed its response to the Complaint and in March 2017 the District Court for the Northern District of Texas approved the stipulated dismissal of all claims in this complaint predicated on Section 712(D) or Section 714(A) of the Louisiana Securities Law.

Another one of the cases, filed in the 23rd Judicial District Court for the Parish of Ascension, also was removed to federal court and transferred by the Judicial Panel on Multidistrict Litigation to the Northern District of Texas and the Stanford MDL. The schedule for responding to that Complaint has not yet been established.

The plaintiffs in two of the cases remaining in the Parish of East Baton Rouge have granted SEI and SPTC indefinite extensions to respond to the petitions.

In the two additional cases, filed in East Baton Rouge and brought by the same counsel who filed the Lillie action, virtually all of the litigation to date has involved motions practice and appellate litigation regarding the existence of federal subjection matter jurisdiction under the federal Securities Litigation Uniform Standards Act (SLUSA). After the matter was

removed to the United States District Court for the Northern District of Texas, that court dismissed the action under SLUSA. The Court of Appeals for the Fifth Circuit reversed that order, and the Supreme Court of the United States affirmed the Court of Appeals judgment on February 26, 2014. The matter was remanded to state court and no material activity has taken place since that date.

While the outcome of this litigation remains uncertain, SEI and SPTC believe that they have valid defenses to plaintiffs' claims and intend to defend the lawsuits vigorously. Because of uncertainty in the make-up of the Lillie class, the specific theories of liability that may survive a motion for summary judgment or other dispositive motion, the relative lack of discovery regarding damages, causation, mitigation and other aspects that may ultimately bear upon loss, the Company is not reasonably able to provide an estimate of loss, if any, with respect to the foregoing lawsuits.

On November 26, 2014, a Writ of Summons was issued to two of our subsidiaries, SEI Investments - Global Fund Services Limited (GFSL) and SEI Investments - Depositary & Custodial Services (Ireland) Limited (D&C), to appear before the Court of First Instance Antwerp, Belgium. The plaintiffs in this case allege that through their initial investments in collective investment funds domiciled in Netherlands and subsequent transfer of claim rights to a Belgium domiciled partnership, they are beneficial owners of a portfolio of life settlement policies (the Portfolio) which lapsed due to a failure to make premium payments. The plaintiffs seek to recover jointly and severally from nine defendants including GFSL and D&C, damages of approximately $84 million. GFSL and D&C’s involvement in the litigation appears to arise out of their historical provision of administration and custody services, respectively, to the Strategic Life Settlement Fund PLC, who, together with its managers, appear to be the principal defendants in this claim. On December 4, 2015, the Belgium court dismissed plaintiff's claims for a lack of jurisdiction. On December 22, 2015, the plaintiffs appealed the dismissal. During October 2017, the Belgium appellate court dismissed plaintiff's appeal.

While the outcome of this action is uncertain given its early phase and the lack of specific theories of liability asserted against GFSL and D&C, each of GFSL and D&C believe that they have valid defenses to plaintiffs’ claims and intend to defend the lawsuit vigorously, and GFSL and D&C are not reasonably able to provide an estimate of the ultimate loss, if any, with respect to this lawsuit.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

The federal and state and foreign income tax provision is summarized as follows:



Year Ended December 31,	2017			2016			2015
Current
Federal	$	154,776		$	158,411		$	159,774
State	11,645			10,500			7,756
Foreign	8,002			5,137			5,224
	174,423			174,048			172,754
Deferred
Federal	(26,350		)	788			(5,343		)
State	1,378			(168		)	1,414
Foreign	3,199			—			—
	(21,773		)	620			(3,929		)
Total income taxes	$	152,650		$	174,668		$	168,825

Annual tax provisions include amounts considered sufficient to pay assessments that may result from examination of prior year tax returns; however, the amount ultimately paid upon resolution of issues raised may differ materially from the amount accrued. The examination and the resolution process may last longer than one year.

The components of Income before income taxes are summarized as follows:



Year Ended December 31,	2017		2016		2015
Domestic	$	523,044	$	481,760	$	472,384
Foreign	33,995		26,725		28,096
	$	557,039	$	508,485	$	500,480

The effective income tax rate differs from the federal income tax statutory rate due to the following:



Year Ended December 31,	2017		2016		2015
Statutory rate	35.0	%	35.0	%	35.0	%
State taxes, net of federal tax benefit	1.3		1.3		1.6
Foreign tax expense and tax rate differential	(1.1	)	(0.8	)	(1.2	)
Tax benefit from stock option exercises	(3.9	)	—		—
Enactment of the Tax Cuts and Jobs Act:
Re-measurement of deferred tax liability	(4.9	)	—		—
One-time transition tax on repatriation of foreign earnings and withholding tax	2.6		—		—
Research and development tax credit	(0.9	)	(0.8	)	(0.6	)
Domestic Production Activities Deduction	(0.5	)	(0.6	)	(0.6	)
Settlement of state tax petition	—		—		(0.8	)
Other, net	(0.2	)	0.2		0.3
	27.4	%	34.3	%	33.7	%

The decrease in the Company's effective income tax rate in 2017 was primarily due to the adoption of ASU 2016-09 and the estimated impact of the Tax Cuts and Jobs Act (Tax Act). As required by ASU 2016-09, the Company no longer records excess tax benefits from stock option exercises as an increase to additional paid in capital, but records such excess tax benefits as a reduction of income tax expense in the reporting period in which the exercises occur.

The Tax Act was enacted on December 22, 2017 and included numerous changes to the current tax law including a permanent reduction in the corporate tax rate from 35.0 percent to 21.0 percent, as well as the imposition of a territorial rather than worldwide system which requires a one-time transition tax on the repatriation of previously deferred foreign earnings. The Company's estimate of the one-time transition tax as of December 31, 2017 was $11,544, of which $915 is expected to be paid within one year and $10,629 is included in Long-term income taxes payable on the accompanying Consolidated Balance Sheet.

Under the Tax Act, the undistributed earnings of the Company’s foreign subsidiaries are deemed repatriated. As a result, the Company has determined that a portion of these earnings are no longer indefinitely reinvested and therefore accrued foreign withholding taxes. The Company's foreign withholding tax was $3,199 and is included in Deferred income taxes on the accompanying Consolidated Balance Sheet as of December 31, 2017.

The impact to the Company's effective tax rate in 2017 from the Tax Act was a combination of a $27,153 tax benefit from the re-measurement of the Company's estimated net deferred tax liability as of December 31, 2017 based upon the new 21.0 percent corporate tax rate offset by expense of $14,743 from the preliminary estimate of the one-time transition tax relating to the impact of the deemed repatriation and withholding tax of the Company's previously undistributed foreign earnings. The net impact to the Company's tax rate in 2017 from the Tax Act was a net tax benefit of $12,410, or $0.08 diluted earnings per share.

The increase in the Company's tax rate for 2016 was primarily due to lower foreign income in jurisdictions with lower effective rates and the one-time reduction in 2015 due to a favorable settlement of a state tax petition.

The SEC staff issued Staff Accounting Bulletin No. 118 (SAB 118) to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Act and allows the registrant to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company's preliminary estimate of the one-time transition tax and the remeasurement of deferred tax assets and liabilities is subject to the finalization of management’s analysis related to certain matters, such as developing interpretations of the provisions of the Tax Act, changes to certain estimates and amounts related to the earnings and profits of certain subsidiaries and the filing of our tax returns. U.S. Treasury regulations, administrative interpretations or court decisions interpreting the Tax Act may require further adjustments and changes in estimates. The final determination of the one-time transition tax and the remeasurement of the Company's deferred assets and liabilities will be completed as additional information becomes available, but no later than one year from the enactment of the 2017 Tax Act in accordance with SAB 118.

Deferred income taxes for 2017, 2016 and 2015 reflect the impact of temporary differences between the amount of assets and liabilities for financial reporting purposes and such amounts as measured by tax laws and regulations. Significant components of our deferred tax assets and liabilities at December 31, 2017 and 2016 are as follows:



	2017			2016
Deferred Tax Assets:
Stock-based compensation expense	$	24,725		$	33,459
Foreign and state net operating loss carryforward	71,236			20,049
Basis differences in investments	4,191			6,165
Federal benefit of state tax deduction for uncertain tax positions	1,918			3,647
Revenue and expense recognized in different periods for financial reporting and income tax purposes	2,631			5,022
Other assets	273			813
Total deferred income tax assets	104,974			69,155
Less: valuation allowance	(68,469		)	(17,922		)
Net deferred income tax assets	$	36,505		$	51,233

Deferred Tax Liabilities:
Capitalized software currently deductible for tax purposes, net of amortization	$	(70,575	)	$	(107,897	)
Difference in financial reporting and income tax depreciation methods	(3,182		)	(5,190		)
Difference between book and tax basis of other assets	(3,549		)	(4,597		)
Goodwill and other intangibles	(1,001		)	—
Foreign Dividend Withholding Tax	(3,199		)	—
Other liabilities	(704		)	(1,115		)
Total deferred income tax liabilities	$	(82,210	)	$	(118,799	)
Net deferred income tax liabilities	$	(45,705	)	$	(67,566	)

The valuation allowances against deferred tax assets at December 31, 2017 and 2016 are related to state net operating losses from certain domestic subsidiaries as well as foreign net operating losses from certain foreign subsidiaries. Certain state and foreign tax statutes significantly limit the utilization of net operating losses for domestic and foreign subsidiaries. Furthermore, these net operating losses cannot be used to offset the net income of other subsidiaries.

The Company recognizes uncertain tax positions in accordance with the applicable accounting guidance and adjusts these liabilities when management’s judgment changes as a result of the evaluation of new information not previously available. Due to the complexity of some of these uncertainties, the ultimate resolution may result in a payment that is materially different from our current estimate of the tax liabilities. The Company’s total unrecognized tax benefit, not including interest and penalties, as of December 31, 2017 was $14,480, of which $13,737 would affect the effective tax rate if the Company were to recognize the tax benefit. The gross amount of uncertain tax liability of $3,275 which is expected to be paid within one year is netted against the current payable account while the remaining amount of $12,380 is included in Other long-term liabilities on the accompanying Consolidated Balance Sheets. During the year ended December 31, 2017, the Company recognized $5,846 of previously unrecognized tax benefits relating to the lapse of the statute of limitation.

The Company files a consolidated federal income tax return and separate income tax returns with various states. Certain subsidiaries of the Company file tax returns in foreign jurisdictions. The Company is no longer subject to U.S. federal income tax examination for years before 2014 and is no longer subject to state, local or foreign income tax examinations by authorities for years before 2010.

A reconciliation of the beginning and ending amount of unrecognized tax benefit is as follows:



	2017			2016			2015
Balance as of January 1	$	17,287		$	14,517		$	14,018
Tax positions related to current year:
Gross additions	3,180			3,756			1,954
Tax positions related to prior years:
Gross additions	211			1,762			297
Settlements	(352		)	(378		)	—
Lapses on statute of limitations	(5,846		)	(2,370		)	(1,752		)
Balance as of December 31	$	14,480		$	17,287		$	14,517

The above reconciliation of the gross unrecognized tax benefit will differ from the amount which would affect the effective tax rate because of the recognition of the federal and state tax benefits.

The Company classifies all interest and penalties as income tax expense. The Company has recorded $1,175, $1,227 and $1,391 in liabilities for tax-related interest and penalties in 2017, 2016 and 2015, respectively.

The Company estimates it will recognize $3,275 of unrecognized tax benefits within the next twelve months due to lapses on the statute of limitation.

The Company includes its direct and indirect subsidiaries in its U.S. consolidated federal income tax return. The Company’s tax sharing allocation agreement provides that any subsidiary having taxable income will pay a tax liability equivalent to what that subsidiary would have paid if it filed a separate income tax return. If the separately calculated federal income tax provision for any subsidiary results in a tax loss, the current benefit resulting from such loss, to the extent utilizable on a separate return basis, is accrued and paid to that subsidiary.

Business Segment Information

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

Business Segment Information

The Company’s reportable business segments are:

Private Banks – provides outsourced investment processing and investment management platforms to banks and trust institutions, independent wealth advisers and financial advisors worldwide;

Investment Advisors – provides investment management and investment processing platforms to affluent investors through a network of independent registered investment advisors, financial planners and other investment professionals in the United States;

Institutional Investors – provides investment management and administrative outsourcing platforms to retirement plan sponsors, healthcare systems and not-for-profit organizations worldwide;

Investment Managers – provides investment operations outsourcing platforms to fund companies, banking institutions and both traditional and non-traditional investment managers worldwide; and

Investments in New Businesses – focuses on providing investment management programs to ultra-high-net-worth families residing in the United States; developing internet-based investment services and advice platforms; entering new markets; and conducting other research and development activities.

In 2017, 2016 and 2015, no single customer accounted for more than ten percent of revenues in any business segment.

The following tables highlight certain financial information about each of the Company’s business segments for the years ended December 31, 2017, 2016 and 2015:



	Private Banks		Investment Advisors		Institutional Investors		Investment Managers		Investments In New Businesses			Total
	For the Year Ended December 31, 2017
Revenues	$	474,272	$	373,473	$	322,457	$	349,444	$	6,906		$	1,526,552
Expenses	455,119		201,833		161,640		226,504		20,678			1,065,774
Operating profit (loss)	$	19,153	$	171,640	$	160,817	$	122,940	$	(13,772	)	$	460,778



	Private Banks		Investment Advisors		Institutional Investors		Investment Managers		Investments In New Businesses			Total
	For the Year Ended December 31, 2016
Revenues	$	457,886	$	330,677	$	312,584	$	294,390	$	6,008		$	1,401,545
Expenses	421,188		180,140		153,117		191,127		20,962			966,534
Operating profit (loss)	$	36,698	$	150,537	$	159,467	$	103,263	$	(14,954	)	$	435,011
Gain on sale of subsidiary	2,791		—		—		—		—			2,791
Total profit (loss)	$	39,489	$	150,537	$	159,467	$	103,263	$	(14,954	)	$	437,802



	Private Banks		Investment Advisors		Institutional Investors		Investment Managers		Investments In New Businesses			Total
	For the Year Ended December 31, 2015
Revenues	$	456,516	$	306,620	$	297,568	$	267,963	$	5,541		$	1,334,208
Expenses	410,975		171,968		145,851		172,094		20,656			921,544
Operating profit (loss)	$	45,541	$	134,652	$	151,717	$	95,869	$	(15,115	)	$	412,664
Gain on sale of subsidiary	2,791		—		—		—		—			2,791
Total profit (loss)	$	48,332	$	134,652	$	151,717	$	95,869	$	(15,115	)	$	415,455

A reconciliation of the total reported for the business segments to income from operations in the Consolidated Statements of Operations for the years ended December 31, 2017, 2016 and 2015 is as follows:



Year Ended December 31,	2017			2016			2015
Total operating profit from segments above	$	460,778		$	435,011		$	412,664
Corporate overhead expenses	(63,834		)	(59,317		)	(54,451		)
Income from operations	$	396,944		$	375,694		$	358,213

The following tables provide additional information for the years ended December 31, 2017, 2016 and 2015 pertaining to our business segments:



	Capital Expenditures (1)						Depreciation
Year Ended December 31,	2017		2016		2015		2017		2016		2015
Private Banks	$	47,526	$	45,940	$	41,972	$	16,479	$	13,222	$	12,348
Investment Advisors	17,450		17,610		13,206		3,364		3,880		3,410
Institutional Investors	4,020		4,319		5,301		1,121		1,367		1,200
Investment Managers	15,863		11,209		10,119		4,698		4,877		4,040
Investments in New Businesses	546		726		736		826		2,197		2,278
Total from business segments	$	85,405	$	79,804	$	71,334	$	26,488	$	25,543	$	23,276
Corporate Overhead	1,163		1,985		2,547		823		897		768
	$	86,568	$	81,789	$	73,881	$	27,311	$	26,440	$	24,044

(1) Capital expenditures include additions to property and equipment and capitalized software.



	Amortization
Year Ended December 31,	2017		2016		2015
Private Banks	$	32,696	$	31,675	$	29,819
Investment Advisors	10,930		10,458		9,880
Institutional Investors	1,599		1,674		1,558
Investment Managers	2,593		1,092		1,029
Investments in New Businesses	239		146		116
Total from business segments	$	48,057	$	45,045	$	42,402
Corporate Overhead	218		347		228
	$	48,275	$	45,392	$	42,630



	Total Assets
	2017		2016
Private Banks	$	523,214	$	471,435
Investment Advisors	139,697		160,717
Institutional Investors	117,286		109,879
Investment Managers	278,540		158,621
Investments in New Businesses	19,269		4,910
Total from business segments	$	1,078,006	$	905,562
Corporate Overhead (2)	775,363		731,261
	$	1,853,369	$	1,636,823

(2) Unallocated assets primarily consist of cash and cash equivalents, marketable securities, and certain other shared services assets.

The following table presents revenues based on the location of the use of the products or services:



For the Year Ended December 31,	2017		2016		2015
United States	$	1,298,381	$	1,191,640	$	1,123,165
International operations	228,171		209,905		211,043
	$	1,526,552	$	1,401,545	$	1,334,208

The following table presents assets based on their location:



	2017		2016
United States	$	1,471,260	$	1,317,235
International operations	382,109		319,588
	$	1,853,369	$	1,636,823

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The Company, either by itself or through its wholly-owned subsidiaries, serves as the sponsor, administrator, investment advisor, distributor and shareholder servicer for SEI-sponsored investment products. These investment products are offered to clients of the Company and its subsidiaries. Fees earned by the Company for the related services are recognized pursuant to the provisions of investment advisory, fund administration, distribution, and shareholder services agreements directly with the investment products. These fees totaled $453,438, $431,318 and $426,301 in 2017, 2016 and 2015, respectively, and are reflected in Asset management, administration and distribution fees on the accompanying Consolidated Statements of Operations. The Company also serves as an introducing broker-dealer for securities transactions of SEI-sponsored investment products. The Company recognized $1,216, $561 and $365 in commissions during 2017, 2016 and 2015, respectively. These fees are reflected in Transaction-based and trade execution fees on the accompanying Consolidated Statements of Operations. Receivables from investment products on the accompanying Consolidated Balance Sheets primarily represent fees receivable for distribution, investment advisory, and administration services to various investment products sponsored by SEI.

Sale of SEI Asset Korea	12 Months Ended
Sale of SEI Asset Korea	Dec. 31, 2017
Discontinued Operations and Disposal Groups [Abstract]
Sale of SEI Asset Korea	Sale of SEI Asset Korea On July 31, 2012, the Company, MetLife International Holdings, Inc. (MetLife) and International Finance Corporation (IFC) entered into a definitive agreement with Baring Asset Management Limited (Barings) to sell all ownership interest in SEI Asset Korea (SEI AK). SEI AK was located in South Korea and provided domestic equity and fixed-income investment management services to financial institutions and pension funds. On March 28, 2013, all conditions subject to closing the transaction were satisfied and all ownership interests in SEI AK were transferred to Barings. Under the terms of the agreement, a portion of the purchase price was paid upon closing with up to an additional $11,220 payable to the Company as a contingent purchase price with respect to three one-year periods ending on December 31, 2013, 2014 and 2015 depending upon whether SEI AK achieves specified revenue measures during such periods. The Company recognized a pre-tax gain of $2,791, or $0.01 diluted earnings per share, during 2015 and 2016. The Company's gains from the sale of SEI AK are included in Gain on sale of subsidiary on the accompanying Consolidated Statement of Operations.

Business Acquisitions Business Acquisitions

12 Months Ended

Dec. 31, 2017

Business Acquisitions [Abstract]

Business Acquisitions

On July 3, 2017, the Company acquired all ownership interests of Archway Technology Partners, LLC, Archway Finance & Operations, Inc. and Keystone Capital Holdings, LLC (collectively, Archway), a provider of operating technologies and services to the family office industry, from Keystone International Holdings, Inc. With this acquisition, the Company expands its position in the single and multi-family office services market by diversifying its technology and operating solutions.

Under the acquisition method of accounting, the total purchase price was allocated to Archway's net tangible and intangible assets based upon their estimated fair values as of July 3, 2017. The total purchase price for Archway was $81,635 in cash consideration with up to an additional $8,000 payable to the seller as a contingent purchase price with respect to two one-year periods ending December 31, 2017 and 2018 depending upon whether Archway achieves specified financial measures during such periods. The fair value of the contingent consideration was estimated to be $4,800 on the acquisition date. Archway did not attain the specified financial measure for the period ended December 31, 2017. As a result, as of December 31, 2017, the Company reversed the value allocated to the contingent consideration established at the acquisition date. Additionally, the Company recognized a liability for post-acquisition obligations to the members of Archway. The net adjustment of $3,800 was recorded as a reduction in expense and is reflected in Facilities, supplies and other costs on the accompanying Consolidated Statement of Operations.

The Company acquired $1,401 in cash during the acquisition, resulting in $80,234 net cash paid for Archway. According to the terms of the purchase agreement, a portion of the purchase price was placed into escrow to indemnify the Company of any pre-acquisition damages. As of December 31, 2017, the balance available in escrow was $8,000.

The purchase price allocation is as follows:



	Estimated Fair Value			Estimated Useful Life
Current assets, net of current liabilities	$	2,539
Property and equipment	776
Goodwill	52,990
Identifiable intangible assets
Acquired technology	13,510			10 years
Client relationships	10,760			15 years
Non-competition agreements	3,470			5 years
Trade names	2,390			7 years
Contingent consideration	(4,800		)
Total purchase price allocation	$	81,635

The results of operations of Archway and any adjustments related to the acquisition are included in the Investment Managers business segment and are reflected in the Company's Consolidated Statement of Operations since the completion of the acquisition on July 3, 2017.

All tangible and intangible assets resulting from the Archway transaction have been allocated to the Investment Managers business segment. Amortization expense related to the intangible assets acquired was $1,552 during the period ended December 31, 2017. The Company currently expects to recognize approximately $3,104 in amortization expense related to the intangible assets each year from 2018 through 2021 and $2,757 in expense in 2022. Any goodwill generated from the acquisition is fully deductible for income tax purposes.

Pro forma information has not been presented because the effect of the Archway acquisition is not material to the Company's consolidated financial results.

Quarterly Financial Data

12 Months Ended

Dec. 31, 2017

Quarterly Financial Data [Abstract]

Quarterly Financial Data

Quarterly Financial Data (Unaudited)



	For the Three Months Ended
2017	March 31			June 30			September 30			December 31(1)
Revenues	$	359,984		$	372,331		$	386,018		$	408,219
Income before income taxes	$	128,660		$	135,158		$	140,769		$	152,452
Net income	$	88,737		$	91,769		$	101,739		$	122,144
Basic earnings per share	$	0.56		$	0.58		$	0.64		$	0.78
Diluted earnings per share (2)	$	0.55		$	0.57		$	0.63		$	0.75

Effective income tax rate	31.0		%	32.1		%	27.7		%	19.9		%

(1) During the fourth quarter 2017, the Company recognized an estimated net tax benefit of $12,410, or $0.08 diluted earnings per share, resulting from the impacts of the enactment of the Tax Act on December 22, 2017 (See Note 12).

(2) The sum of the individual quarterly earnings per share amounts may not agree with the annual earnings per share amount as each quarterly computation is based on the weighted average number of shares outstanding during that period.



	For the Three Months Ended
2016	March 31			June 30			September 30			December 31(3)
Revenues	$	334,263		$	343,831		$	354,641		$	368,810
Income before income taxes	$	119,638		$	124,904		$	130,890		$	133,053
Net income	$	77,497		$	81,005		$	86,704		$	88,611
Basic earnings per share	$	0.48		$	0.50		$	0.54		$	0.55
Diluted earnings per share (2)	$	0.47		$	0.49		$	0.53		$	0.55

Effective income tax rate	35.2		%	35.2		%	33.8		%	33.4		%

(3) During the fourth quarter 2016, the Company recognized a performance fee of $12,286 and a corresponding sub-advisory expense of $6,143 associated with an SEI-sponsored investment product. These items resulted in an after-tax net profit of $4,091, or $0.03 diluted earnings per share.

Schedule II - Valuation And Qualifying Accounts And Reserves

12 Months Ended

Dec. 31, 2017

Valuation and Qualifying Accounts [Abstract]

Schedule II - Valuation and Qualifying Accounts and Reserves



Year Ended December 31,			Additions
Description	Balance at Beginning of Year		Charged to Costs and Expenses			Charged to Other Accounts			(Deductions)			Balance at End of Year
Allowance for doubtful accounts:
2017	$	523	$	172		$	—		$	—		$	695
2016	649		—			—			(126		)	523
2015	784		—			—			(135		)	649
Deferred income tax valuation allowance:
2017 (1)	$	17,922	$	—		$	50,547		$	—		$	68,469
2016	14,548		—			3,374			—			17,922
2015	16,509		(1,142		)	(819		)	—			14,548

(1) Additions to deferred income tax valuation allowance in 2017 reflect the Pennsylvania state tax law change in October 2017 regarding the use of net operating losses.

Summary of Significant Accounting Policies (Policy)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Principles of Consolidation

Variable Interest Entities

Management's Use of Estimates

Management’s Use of EstimatesThe preparation of financial statements in conformity with accounting principles generally accepted in the United States (U.S. GAAP) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

Revenue Recognition

Cash and Cash Equivalents

Cash and Cash EquivalentsThe Company considers investment instruments purchased with an original maturity of three months or less to be cash equivalents.

Allowance for Doubtful Accounts

Allowances for Doubtful AccountsThe Company provides an allowance for doubtful accounts equal to the estimated uncollectible amounts. The Company’s estimate is based on historical collection experience and a review of the current status of trade accounts receivable.

Concentration of Credit Risk

Concentration of Credit RiskFinancial instruments which potentially expose the Company to concentrations of credit risk consist primarily of cash equivalents and trade receivables. Cash equivalents are principally invested in short-term money market funds or placed with major banks and high-credit qualified financial institutions. Cash deposits maintained with institutions are in excess of federally insured limits. Concentrations of credit risk with respect to the Company's receivables are limited due to the large number of clients and their dispersion across geographic areas. No single group or customer represents greater than ten percent of total accounts receivable.

Property and Equipment

Property and EquipmentProperty and Equipment are recorded at cost. Expenditures for major additions and improvements are capitalized and minor replacements, maintenance, and repairs are charged to expense as incurred. Construction in progress includes the cost of construction and other direct costs attributable to the construction. When property and equipment are retired or disposed of, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations for the respective period. Depreciation is provided over the estimated useful lives using the straight line method for financial statement purposes. No provision for depreciation is made for construction in progress until such time as the relevant assets are completed and put into service. The Company uses other depreciation methods, generally accelerated, for tax purposes where appropriate. Buildings and building improvements are depreciated over 25 to 39 years. Equipment, purchased software and furniture and fixtures have useful lives ranging from 3 to 5 years. Amortization of leasehold improvements is computed using the straight line method over the shorter of the remaining lease term or the estimated useful lives of the improvements.

Marketable Securities

Fair Value of Financial Instruments

Capitalized Software

costs are amortized on a product-by-product basis using the straight-line method over the estimated economic life of the product or enhancement.



2018	$	43,717
2019	43,717
2020	43,717
2021	43,439
2022	29,556

Business Combinations

Business CombinationsThe Company accounts for business combinations in accordance with Accounting Standards Codification Topic 805, Business Combinations (ASC 805). ASC 805 establishes principles and requirements for recognizing the total consideration transferred, assets acquired and liabilities assumed in a business combination. ASC 805 also provides guidance for recognizing and measuring goodwill acquired in a business combination and requires the acquirer to disclose information needed to evaluate and understand the financial impact of the business combination. The Company recognizes assets and liabilities acquired at their estimated fair values. Management uses judgment to identify the acquired assets and liabilities assumed; estimate the fair value of these assets and liabilities; estimate the useful life of the assets; and assess the appropriate method for recognizing depreciation or amortization expense over the estimated useful life of the assets.

Goodwill and Other Intangible Assets

Income Taxes

Income TaxesThe Company applies the asset and liability approach to account for income taxes whereby deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled.As required by ASU 2016-09, the Company no longer records excess tax benefits from stock option exercises as an increase to additional paid in capital, but records such excess tax benefits as a reduction of income tax expense in the reporting period in which the exercises occur.

Foreign Currency Translation

Transaction gains and losses from exchange rate fluctuations are included in the results of operations in the periods in which they occur.

Earnings Per Share

Earnings Per Common ShareBasic earnings per common share is computed by dividing net income attributable to SEI Investments common shareholders by the weighted average number of common shares outstanding during the period. Diluted earnings per common share is computed by dividing net income attributable to SEI Investments common shareholders by the combination of the weighted average number of common shares outstanding and the dilutive potential common shares, such as stock options, outstanding during the period.

Stock-based Compensation

Stock-Based Compensation

New Accounting Pronouncements

consumes the benefits continuously over time. The timing and recognition of revenues from these arrangements will not change as a result of the adoption of ASU 2014-09.

Reclassifications

ReclassificationsCertain prior year amounts have been reclassified to conform to current year presentation.

Investments in Equity Method Investments

The Company accounts for its interest in LSV using the equity method because of its less than 50 percent ownership. The Company’s interest in the net assets of LSV is reflected in Investment in unconsolidated affiliates on the accompanying Consolidated Balance Sheets and its interest in the earnings of LSV is reflected in Equity in earnings of unconsolidated affiliates on the accompanying Consolidated Statements of Operations.

Investments in Affiliated Funds

Securities Owned

Accumulated Other Comprehensive Income (Loss)

Uncertain Tax Positions

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Amortization Expense Related to the SEI Wealth Platform



2018	$	43,717
2019	43,717
2020	43,717
2021	43,439
2022	29,556

Calculation Of Basic And Diluted Earnings Per Share

The calculations of basic and diluted earnings per share for 2017, 2016 and 2015 are:



	2017		2016		2015
Net income	$	404,389	$	333,817	$	331,655
Shares used to compute basic earnings per common share	158,177,000		161,350,000		165,725,000
Dilutive effect of stock options	4,092,000		3,081,000		3,873,000
Shares used to compute diluted earnings per common share	162,269,000		164,431,000		169,598,000
Basic earnings per common share	$	2.56	$	2.07	$	2.00
Diluted earnings per common share	$	2.49	$	2.03	$	1.96

Composition of Certain Financial Statement Captions (Tables)

12 Months Ended

Dec. 31, 2017

Items Included in Consolidated Statement of Financial Condition [Abstract]

Receivables

Receivables on the accompanying Consolidated Balance Sheets consist of:



	2017			2016
Trade receivables	$	76,760		$	48,683
Fees earned, not billed	194,331			168,971
Other receivables	12,310			10,826
	283,401			228,480
Less: Allowance for doubtful accounts	(695		)	(523		)
Receivables, net	$	282,706		$	227,957

Property And Equipment

Property and Equipment on the accompanying Consolidated Balance Sheets consists of:



	2017			2016
Buildings	$	153,961		$	152,171
Equipment	115,546			106,759
Land	10,030			10,030
Purchased software	134,610			128,008
Furniture and fixtures	18,114			17,292
Leasehold improvements	18,017			15,175
Construction in progress	6,105			2,077
	456,383			431,512
Less: Accumulated depreciation	(309,955		)	(285,322		)
Property and Equipment, net	$	146,428		$	146,190

Accrued Liabilities

Accrued Liabilities on the accompanying Consolidated Balance Sheets consist of:



	2017		2016
Accrued employee compensation	$	88,960	$	79,735
Accrued consulting, outsourcing and professional fees	29,658		24,428
Accrued sub-advisory, distribution and other asset management fees	42,365		41,666
Accrued dividend payable	47,179		44,596
Other accrued liabilities	56,896		50,100
Accrued liabilities	$	265,058	$	240,525

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Of Certain Financial Assets And Liabilities

The fair value of certain financial assets and liabilities of the Company was determined using the following inputs:



			Fair Value Measurements at Reporting Date Using
	December 31, 2017		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
Assets
Equity available-for-sale securities	$	11,250	$	11,250	$	—
Fixed-income available-for-sale securities	76,733		—		76,733
Fixed-income securities owned	21,526		—		21,526
Investment funds sponsored by LSV (1)	6,034
	$	115,543	$	11,250	$	98,259



			Fair Value Measurements at Reporting Date Using
	December 31, 2016		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
Assets
Equity available-for-sale securities	$	9,581	$	9,581	$	—
Fixed-income available-for-sale securities	74,452		—		74,452
Fixed-income securities owned	21,339		—		21,339
Investment funds sponsored by LSV (1)	4,858
	$	110,230	$	9,581	$	95,791

Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2017

Marketable Securities [Abstract]

Investments Available For Sale

Investments available for sale classified as non-current assets consist of:



	At December 31, 2017
	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)			Fair Value
SEI-sponsored mutual funds	$	7,369	$	110	$	(143	)	$	7,336
Equities and other mutual funds	3,456		458		—			3,914
Debt securities	77,745		—		(1,012		)	76,733
	$	88,570	$	568	$	(1,155	)	$	87,983



	At December 31, 2016
	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)			Fair Value
SEI-sponsored mutual funds	$	7,357	$	24	$	(996	)	$	6,385
Equities and other mutual funds	2,968		228		—			3,196
Debt securities	74,843		—		(391		)	74,452
	$	85,168	$	252	$	(1,387	)	$	84,033

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Shareholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Stockholders' Equity Note [Abstract]

Assumptions Used In The Weighted Average Fair Value Of The Stock Options Granted

The weighted average fair value of the Company’s stock options granted during 2017, 2016 and 2015 were $16.78, $12.43 and $12.16, respectively, using the following assumptions:



	2017		2016		2015
Expected term (in years)	6.00		6.00		5.58
Expected volatility	22.58	%	25.44	%	23.86	%
Expected dividend yield	0.82	%	1.10	%	1.00	%
Risk-free interest rate	2.29	%	2.18	%	1.90	%

Stock-based Compensation Expense

The Company recognized stock-based compensation expense in its Consolidated Financial Statements in 2017, 2016 and 2015 as follows:



	2017			2016			2015
Stock-based compensation expense	$	36,366		$	16,017		$	17,312
Less: Deferred tax benefit	(7,891		)	(5,612		)	(6,107		)
Stock-based compensation expense, net of tax	$	28,475		$	10,405		$	11,205

Stock Option Plans

This table presents certain information relating to the Company’s stock option plans for 2017, 2016 and 2015:



	Number of Shares		Weighted Avg. Price
Balance as of December 31, 2014	20,461,000		$	25.26
Granted	2,005,000		53.34
Exercised	(2,927,000	)	21.44
Expired or canceled	(302,000	)	28.97
Balance as of December 31, 2015	19,237,000		$	28.71
Granted	2,310,000		49.57
Exercised	(1,809,000	)	24.82
Expired or canceled	(1,669,000	)	30.86
Balance as of December 31, 2016	18,069,000		$	31.57
Granted	2,057,000		69.87
Exercised	(2,370,000	)	21.22
Expired or canceled	(1,044,000	)	33.42
Balance as of December 31, 2017	16,712,000		$	37.63

Exercisable as of December 31, 2017	10,624,000		$	28.99
Available for future grant as of December 31, 2017	24,108,000

Stock Options Outstanding And Exercisable

This table summarizes information relating to all options outstanding and exercisable at December 31, 2017:



				Options Outstanding at December 31, 2017				Options Exercisable at December 31, 2017
Range of Exercise Prices (Per Share)				Number of Shares	Weighted Average Exercise Price (Per Share)		Weighted Average Remaining Contractual Life (Years)	Number of Shares	Weighted Average Exercise Price (Per Share)		Weighted Average Remaining Contractual Life (Years)
$	14.62	-	15.77	2,178,000	$	15.45	3.16	2,178,000	$	15.45	3.16
17.65		-	21.05	1,174,000	17.67		2.01	1,174,000	17.67		2.01
22.45		-	23.86	3,432,000	23.19		3.95	3,422,000	23.19		3.96
27.03		-	39.15	1,815,000	33.65		6.09	747,000	33.58		6.09
40.64		-	53.16	4,190,000	45.49		8.54	2,095,000	45.49		8.08
53.34		-	71.12	3,923,000	62.01		9.05	1,008,000	53.39		8.14
				16,712,000				10,624,000

Common Stock Buyback

The following table provides the total number of shares repurchased and the related total costs in 2017, 2016 and 2015:



Year	Total Number of Shares Repurchased	Total Cost
2017	4,403,000	$	248,114
2016	6,600,000	294,374
2015	5,951,000	289,587

Accumulated Other Comprehensive Income (Loss), Net of Tax (Tables)

12 Months Ended

Dec. 31, 2017

Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]

Schedule Of Accumulated Other Comprehensive Income (Loss), Net Of Tax

Components of Accumulated other comprehensive income (loss), net of tax, consisted of:



	Foreign Currency Translation Adjustments			Unrealized Holding Gains (Losses) on Investments			Accumulated Other Comprehensive Income (Loss)
Balance, January 1, 2015	$	(10,088	)	$	1,193		$	(8,895	)

Other comprehensive loss before reclassifications	(14,900		)	(1,659		)	(16,559		)
Amounts reclassified from accumulated other comprehensive loss	—			164			164
Net current-period other comprehensive loss	(14,900		)	(1,495		)	(16,395		)

Balance, December 31, 2015	$	(24,988	)	$	(302	)	$	(25,290	)

Other comprehensive loss before reclassifications	(12,131		)	(918		)	(13,049		)
Amounts reclassified from accumulated other comprehensive loss	—			384			384
Net current-period other comprehensive loss	(12,131		)	(534		)	(12,665		)

Balance, December 31, 2016	$	(37,119	)	$	(836	)	$	(37,955	)

Other comprehensive gain before reclassifications	17,597			190			17,787
Amounts reclassified from accumulated other comprehensive loss	—			260			260
Net current-period other comprehensive gain	17,597			450			18,047

Balance, December 31, 2017	$	(19,522	)	$	(386	)	$	(19,908	)

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Federal and State and Foreign Income Tax Provision

The federal and state and foreign income tax provision is summarized as follows:



Year Ended December 31,	2017			2016			2015
Current
Federal	$	154,776		$	158,411		$	159,774
State	11,645			10,500			7,756
Foreign	8,002			5,137			5,224
	174,423			174,048			172,754
Deferred
Federal	(26,350		)	788			(5,343		)
State	1,378			(168		)	1,414
Foreign	3,199			—			—
	(21,773		)	620			(3,929		)
Total income taxes	$	152,650		$	174,668		$	168,825

Components of Net Income Before Income Taxes

The components of Income before income taxes are summarized as follows:



Year Ended December 31,	2017		2016		2015
Domestic	$	523,044	$	481,760	$	472,384
Foreign	33,995		26,725		28,096
	$	557,039	$	508,485	$	500,480

Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate

The effective income tax rate differs from the federal income tax statutory rate due to the following:



Year Ended December 31,	2017		2016		2015
Statutory rate	35.0	%	35.0	%	35.0	%
State taxes, net of federal tax benefit	1.3		1.3		1.6
Foreign tax expense and tax rate differential	(1.1	)	(0.8	)	(1.2	)
Tax benefit from stock option exercises	(3.9	)	—		—
Enactment of the Tax Cuts and Jobs Act:
Re-measurement of deferred tax liability	(4.9	)	—		—
One-time transition tax on repatriation of foreign earnings and withholding tax	2.6		—		—
Research and development tax credit	(0.9	)	(0.8	)	(0.6	)
Domestic Production Activities Deduction	(0.5	)	(0.6	)	(0.6	)
Settlement of state tax petition	—		—		(0.8	)
Other, net	(0.2	)	0.2		0.3
	27.4	%	34.3	%	33.7	%

Schedule of Deferred Tax Assets and Liabilities

Significant components of our deferred tax assets and liabilities at December 31, 2017 and 2016 are as follows:



	2017			2016
Deferred Tax Assets:
Stock-based compensation expense	$	24,725		$	33,459
Foreign and state net operating loss carryforward	71,236			20,049
Basis differences in investments	4,191			6,165
Federal benefit of state tax deduction for uncertain tax positions	1,918			3,647
Revenue and expense recognized in different periods for financial reporting and income tax purposes	2,631			5,022
Other assets	273			813
Total deferred income tax assets	104,974			69,155
Less: valuation allowance	(68,469		)	(17,922		)
Net deferred income tax assets	$	36,505		$	51,233

Deferred Tax Liabilities:
Capitalized software currently deductible for tax purposes, net of amortization	$	(70,575	)	$	(107,897	)
Difference in financial reporting and income tax depreciation methods	(3,182		)	(5,190		)
Difference between book and tax basis of other assets	(3,549		)	(4,597		)
Goodwill and other intangibles	(1,001		)	—
Foreign Dividend Withholding Tax	(3,199		)	—
Other liabilities	(704		)	(1,115		)
Total deferred income tax liabilities	$	(82,210	)	$	(118,799	)
Net deferred income tax liabilities	$	(45,705	)	$	(67,566	)

Schedule of Unrecognized Tax Benefits Roll Forward

A reconciliation of the beginning and ending amount of unrecognized tax benefit is as follows:



	2017			2016			2015
Balance as of January 1	$	17,287		$	14,517		$	14,018
Tax positions related to current year:
Gross additions	3,180			3,756			1,954
Tax positions related to prior years:
Gross additions	211			1,762			297
Settlements	(352		)	(378		)	—
Lapses on statute of limitations	(5,846		)	(2,370		)	(1,752		)
Balance as of December 31	$	14,480		$	17,287		$	14,517

Business Segment Information (Tables)

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

Financial Information About Business Segments

The following tables highlight certain financial information about each of the Company’s business segments for the years ended December 31, 2017, 2016 and 2015:



	Private Banks		Investment Advisors		Institutional Investors		Investment Managers		Investments In New Businesses			Total
	For the Year Ended December 31, 2017
Revenues	$	474,272	$	373,473	$	322,457	$	349,444	$	6,906		$	1,526,552
Expenses	455,119		201,833		161,640		226,504		20,678			1,065,774
Operating profit (loss)	$	19,153	$	171,640	$	160,817	$	122,940	$	(13,772	)	$	460,778



	Private Banks		Investment Advisors		Institutional Investors		Investment Managers		Investments In New Businesses			Total
	For the Year Ended December 31, 2016
Revenues	$	457,886	$	330,677	$	312,584	$	294,390	$	6,008		$	1,401,545
Expenses	421,188		180,140		153,117		191,127		20,962			966,534
Operating profit (loss)	$	36,698	$	150,537	$	159,467	$	103,263	$	(14,954	)	$	435,011
Gain on sale of subsidiary	2,791		—		—		—		—			2,791
Total profit (loss)	$	39,489	$	150,537	$	159,467	$	103,263	$	(14,954	)	$	437,802



	Private Banks		Investment Advisors		Institutional Investors		Investment Managers		Investments In New Businesses			Total
	For the Year Ended December 31, 2015
Revenues	$	456,516	$	306,620	$	297,568	$	267,963	$	5,541		$	1,334,208
Expenses	410,975		171,968		145,851		172,094		20,656			921,544
Operating profit (loss)	$	45,541	$	134,652	$	151,717	$	95,869	$	(15,115	)	$	412,664
Gain on sale of subsidiary	2,791		—		—		—		—			2,791
Total profit (loss)	$	48,332	$	134,652	$	151,717	$	95,869	$	(15,115	)	$	415,455

Reconciliation Of Total Operating Profit Reported For Business Segments To Income From Operations In Consolidated Statements Of Operations



Year Ended December 31,	2017			2016			2015
Total operating profit from segments above	$	460,778		$	435,011		$	412,664
Corporate overhead expenses	(63,834		)	(59,317		)	(54,451		)
Income from operations	$	396,944		$	375,694		$	358,213

Additional Information Pertaining To Business Segments

The following tables provide additional information for the years ended December 31, 2017, 2016 and 2015 pertaining to our business segments:



	Capital Expenditures (1)						Depreciation
Year Ended December 31,	2017		2016		2015		2017		2016		2015
Private Banks	$	47,526	$	45,940	$	41,972	$	16,479	$	13,222	$	12,348
Investment Advisors	17,450		17,610		13,206		3,364		3,880		3,410
Institutional Investors	4,020		4,319		5,301		1,121		1,367		1,200
Investment Managers	15,863		11,209		10,119		4,698		4,877		4,040
Investments in New Businesses	546		726		736		826		2,197		2,278
Total from business segments	$	85,405	$	79,804	$	71,334	$	26,488	$	25,543	$	23,276
Corporate Overhead	1,163		1,985		2,547		823		897		768
	$	86,568	$	81,789	$	73,881	$	27,311	$	26,440	$	24,044

(1) Capital expenditures include additions to property and equipment and capitalized software.



	Amortization
Year Ended December 31,	2017		2016		2015
Private Banks	$	32,696	$	31,675	$	29,819
Investment Advisors	10,930		10,458		9,880
Institutional Investors	1,599		1,674		1,558
Investment Managers	2,593		1,092		1,029
Investments in New Businesses	239		146		116
Total from business segments	$	48,057	$	45,045	$	42,402
Corporate Overhead	218		347		228
	$	48,275	$	45,392	$	42,630



	Total Assets
	2017		2016
Private Banks	$	523,214	$	471,435
Investment Advisors	139,697		160,717
Institutional Investors	117,286		109,879
Investment Managers	278,540		158,621
Investments in New Businesses	19,269		4,910
Total from business segments	$	1,078,006	$	905,562
Corporate Overhead (2)	775,363		731,261
	$	1,853,369	$	1,636,823

(2) Unallocated assets primarily consist of cash and cash equivalents, marketable securities, and certain other shared services assets.

Revenues And Assets Based On Location

The following table presents revenues based on the location of the use of the products or services:



For the Year Ended December 31,	2017		2016		2015
United States	$	1,298,381	$	1,191,640	$	1,123,165
International operations	228,171		209,905		211,043
	$	1,526,552	$	1,401,545	$	1,334,208

The following table presents assets based on their location:



	2017		2016
United States	$	1,471,260	$	1,317,235
International operations	382,109		319,588
	$	1,853,369	$	1,636,823

Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Data [Abstract]

Quarterly Financial Data

Quarterly Financial Data (Unaudited)



	For the Three Months Ended
2017	March 31			June 30			September 30			December 31(1)
Revenues	$	359,984		$	372,331		$	386,018		$	408,219
Income before income taxes	$	128,660		$	135,158		$	140,769		$	152,452
Net income	$	88,737		$	91,769		$	101,739		$	122,144
Basic earnings per share	$	0.56		$	0.58		$	0.64		$	0.78
Diluted earnings per share (2)	$	0.55		$	0.57		$	0.63		$	0.75

Effective income tax rate	31.0		%	32.1		%	27.7		%	19.9		%



	For the Three Months Ended
2016	March 31			June 30			September 30			December 31(3)
Revenues	$	334,263		$	343,831		$	354,641		$	368,810
Income before income taxes	$	119,638		$	124,904		$	130,890		$	133,053
Net income	$	77,497		$	81,005		$	86,704		$	88,611
Basic earnings per share	$	0.48		$	0.50		$	0.54		$	0.55
Diluted earnings per share (2)	$	0.47		$	0.49		$	0.53		$	0.55

Effective income tax rate	35.2		%	35.2		%	33.8		%	33.4		%

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 744,247				$ 695,701				$ 744,247	$ 695,701	$ 679,661	$ 667,446
Restricted cash	3,505				3,500				3,505	3,500
Capitalized software development costs									61,043	50,392	29,416
Net book value of capitalized software	$ 310,405				$ 295,867				310,405	295,867
Increase in income from operations from change in estimate									396,944	375,694	358,213
Increase in net income from change in estimate									$ 404,389	$ 333,817	$ 331,655
Increase in diluted earnings per share from change in estimate	$ 0.75	$ 0.63	$ 0.57	$ 0.55	$ 0.55	$ 0.53	$ 0.49	$ 0.47	$ 2.49	$ 2.03	$ 1.96
Estimated amortization expense related to all capitalized software in 2018	$ 43,717								$ 43,717
Estimated amortization expense related to all capitalized software in 2019	43,717								43,717
Estimated amortization expense related to all capitalized software in 2020	43,717								43,717
Estimated amortization expense related to all capitalizes software in 2021	43,439								43,439
Estimated amortization expense related to all capitalized software in 2022	29,556								$ 29,556
Anti-dilutive employee stock options									5,196	10,632	10,730
Anti-dilutive employee stock options, average exercise price per share									$ 45.49	$ 35.02	$ 33.99
SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs									$ 51,353	$ 39,785	$ 24,515
Net book value of capitalized software	285,207								285,207
Capitalized software development costs in progress	38,093								$ 38,093
Estimated useful life of the SEI Wealth Platform									15 years
Amortization expense related to the SEI Wealth Platform									$ 46,505	45,047	$ 42,401
Income statement classification for capitalized software write off related to the SEI Wealth Platform											Facilities, supplies and other costs
Application for the Investment Managers segment [Member]
Summary of Significant Accounting Policies [Line Items]
Capitalized software development costs									9,690	10,607	$ 4,901
Net book value of capitalized software	25,198								25,198
SEI Investments Europe Limited [Member]
Summary of Significant Accounting Policies [Line Items]
Restricted cash	3,000				$ 3,000				3,000	3,000
SEI Investments Distribution Co [Member]
Summary of Significant Accounting Policies [Line Items]
Restricted cash	$ 505				500				$ 505	500
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Ownership interest in unconsolidated affiliate	50.00%								50.00%
Weighted Average [Member] \| SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of the SEI Wealth Platform	8 years 2 months 12 days
Building and Building Improvements [Member] \| Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment									25 years
Building and Building Improvements [Member] \| Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment									39 years
Computer Equipment [Member] \| Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment									3 years
Computer Equipment [Member] \| Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment									5 years
Furniture and Fixtures [Member] \| Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment									3 years
Furniture and Fixtures [Member] \| Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment									5 years
Equity Method Investee [Member] \| Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Ownership interest in unconsolidated affiliate	20.00%								20.00%
Equity Method Investee [Member] \| Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Ownership interest in unconsolidated affiliate	50.00%								50.00%
Capitalized software [Member] \| SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Loss on write off of capitalized software related to the SEI Wealth Platform											$ 5,533
Private Banks [Member] \| SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Segment classification for capitalized software write off related to the SEI Wealth Platform											Private Banks
Investment Advisors [Member] \| SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Segment classification for capitalized software write off related to the SEI Wealth Platform											Investment Advisors
SEI Sponsored Open Ended Money Market Mutual Funds [Member]
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 401,292				$ 374,760				$ 401,292	$ 374,760
Remaining Useful Life of SEI Wealth Platform [Member] \| SEI Wealth Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Decrease in amortization expense from change in estimate									4,347
Increase in income from operations from change in estimate									4,347
Increase in net income from change in estimate									$ 3,156
Increase in diluted earnings per share from change in estimate									$ 0.02

Summary of Significant Accounting Policies (Calculation Of Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income									$ 404,389	$ 333,817	$ 331,655
Shares used to compute basic earnings per common share									158,177	161,350	165,725
Dilutive effect of stock options									4,092	3,081	3,873
Shares used to compute diluted earnings per common share									162,269	164,431	169,598
Basic earnings per common share	$ 0.78	$ 0.64	$ 0.58	$ 0.56	$ 0.55	$ 0.54	$ 0.50	$ 0.48	$ 2.56	$ 2.07	$ 2.00
Diluted earnings per common share	$ 0.75	$ 0.63	$ 0.57	$ 0.55	$ 0.55	$ 0.53	$ 0.49	$ 0.47	$ 2.49	$ 2.03	$ 1.96

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (New Accounting Pronouncements And Changes In Accounting Principle) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to net cash provided by operating activities	$ 459,898	$ 434,220	$ 407,516
Corresponding offset to net cash used for financing activities	253,633	328,672	305,861
Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to net cash provided by operating activities		8,984	16,056
Corresponding offset to net cash used for financing activities		(8,984)	(16,056)
Increase to deferred tax assets				$ 913
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Research services by SIDCO recorded as expense	14,623	18,409	20,333
Additional Paid-in Capital [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cumulative-effect adjustment	(2,582)	0	0
Additional Paid-in Capital [Member] \| Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cumulative-effect adjustment				(2,582)
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cumulative-effect adjustment	$ 1,669	$ 0	$ 0
Retained Earnings [Member] \| Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cumulative-effect adjustment				$ 1,669

Investment In Unconsolidated Affiliates (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2012
Investments in and Advances to Affiliates [Line Items]
Company's share in the earnings of LSV	$ 152,550	$ 126,103	$ 137,057
Distributions received from LSV	$ 143,517	125,224	141,767
LSV Asset Management [Member]
Investments in and Advances to Affiliates [Line Items]
Ownership interest in LSV	38.90%
Total investment in LSV	$ 59,492
Company's share in the earnings of LSV	152,550	126,103	138,407
Distributions received from LSV	$ 143,517	$ 125,224	$ 141,767
Maximum [Member]
Investments in and Advances to Affiliates [Line Items]
Ownership interest in LSV	50.00%
LSV Asset Management [Member] \| Financial Guarantee [Member]
Investments in and Advances to Affiliates [Line Items]
Maximum exposure from guaranty of LSV Employee Group III obligations				$ 24,930
LSV Employee Group III [Member]
Investments in and Advances to Affiliates [Line Items]
Purchase price of partnership interest in LSV				77,700
Loans payable by LSV Employee Group III				69,930
LSV Employee Group III [Member] \| Financial Guarantee [Member]
Investments in and Advances to Affiliates [Line Items]
Maximum exposure from guaranty of LSV Employee Group III obligations				$ 45,000

Investment In Unconsolidated Affiliates (Condensed Statements of Operations of LSV) (Details) - LSV Asset Management [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Revenues	$ 491,872	$ 399,462	$ 427,653
Net income	$ 392,141	$ 323,381	$ 352,845

Investment In Unconsolidated Affiliates (Condensed Balance Sheets of LSV) (Details) - LSV Asset Management [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Current assets	$ 155,239	$ 125,872
Non-current assets	1,407	1,927
Total assets	156,646	127,799
Current liabilities	46,486	39,303
Partners’ capital	110,160	88,496
Total liabilities and partners’ capital	$ 156,646	$ 127,799

Variable Interest Entities - Investment Products (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEI Sponsored Open Ended Money Market Mutual Funds [Member]
Financial Support for Nonconsolidated Legal Entity [Line Items]
Fees waived according to expense limitation agreements	$ 27,434	$ 41,227	$ 55,713

Composition of Certain Financial Statement Captions (Receivables) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Items Included in Consolidated Statement of Financial Condition [Abstract]
Trade receivables	$ 76,760	$ 48,683
Fees earned, not billed	194,331	168,971
Other receivables	12,310	10,826
Gross receivables	283,401	228,480
Less: Allowance for doubtful accounts	(695)	(523)
Receivables, net	$ 282,706	$ 227,957

Composition of Certain Financial Statement Captions (Property And Equipment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	$ 456,383	$ 431,512
Less: Accumulated depreciation	(309,955)	(285,322)
Property and Equipment, net	146,428	146,190
Depreciation expense	27,311	26,440	$ 24,044
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	153,961	152,171
Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	115,546	106,759
Land [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	10,030	10,030
Purchased software [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	134,610	128,008
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	18,114	17,292
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	18,017	15,175
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	$ 6,105	$ 2,077

Composition of Certain Financial Statement Captions (Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Items Included in Consolidated Statement of Financial Condition [Abstract]
Accrued employee compensation	$ 88,960	$ 79,735
Accrued consulting, outsourcing and professional fees	29,658	24,428
Accrued sub-advisory, distribution and other asset management fees	42,365	41,666
Accrued dividend payable	47,179	44,596
Other accrued liabilities	56,896	50,100
Total accrued liabilities	$ 265,058	$ 240,525

Fair Value Measurements (Fair Value Of Certain Financial Assets And Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity available-for-sale securities	$ 11,250	$ 9,581
Fixed income available-for-sale securities	76,733	74,452
Fixed income securities owned	21,526	21,339
Investment funds sponsored by LSV	6,034	4,858
Assets, fair value	115,543	110,230
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity available-for-sale securities	11,250	9,581
Fixed income available-for-sale securities	0	0
Fixed income securities owned	0	0
Assets, fair value	11,250	9,581
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity available-for-sale securities	0	0
Fixed income available-for-sale securities	76,733	74,452
Fixed income securities owned	21,526	21,339
Assets, fair value	$ 98,259	$ 95,791

Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Net unrealized holding losses	$ 386	$ 836
Income tax benefit from unrealized holding losses during the period	201	299
Gross realized gains from available-for-sale securities	529	284	$ 489
Gross realized losses from available-for-sale securities	888	869	729
Investment funds sponsored by LSV	6,034	4,858
Securities owned	21,526	21,339
Investment funds sponsored by LSV [Member]
Schedule of Investments [Line Items]
Investment funds sponsored by LSV	6,034	4,858
Change in fair value of investment funds sponsored by LSV	$ 1,176	$ 819	$ (389)

Marketable Securities (Investments Available For Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Investment Available for Sale, Cost Amount	$ 88,570	$ 85,168
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	568	252
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1,155)	(1,387)
Available-for-sale Securities	87,983	84,033
SEI-sponsored mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Available for Sale, Cost Amount	7,369	7,357
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	110	24
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(143)	(996)
Available-for-sale Securities	7,336	6,385
Equities and other mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Available for Sale, Cost Amount	3,456	2,968
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	458	228
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities	3,914	3,196
Debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Available for Sale, Cost Amount	77,745	74,843
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1,012)	(391)
Available-for-sale Securities	$ 76,733	$ 74,452

Line Of Credit (Details) - USD ($) $ in Thousands				12 Months Ended
Line Of Credit (Details) - USD ($) $ in Thousands	Jan. 10, 2018	Oct. 10, 2017	Jun. 13, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2018
Line of Credit Facility [Line Items]
Credit facility principal payment				$ 10,000	$ 0	$ 0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility maximum borrowing capacity			$ 300,000
Credit facility stated percentage			0.00%
Credit facility accordion feature, increase limit			$ 100,000
Credit facility borrowings				40,000
Credit facility amount outstanding				30,000
Credit facility principal payment		$ 10,000
Credit facility commitment fees				$ 781	$ 531	$ 483
Revolving Credit Facility [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Credit facility variable interest rate basis spread (as a percent)			1.00%
Credit facility description of variable rate basis			London InterBank Offered Rate
Revolving Credit Facility [Member] \| Federal Funds Rate [Member]
Line of Credit Facility [Line Items]
Credit facility variable interest rate basis spread (as a percent)			0.50%
Credit facility description of variable rate basis			Federal Funds Rate
Minimum [Member] \| Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility commitment fee per annum on daily unused portion (as a percent)			0.15%
Minimum [Member] \| Revolving Credit Facility [Member] \| Base Rate [Member]
Line of Credit Facility [Line Items]
Credit facility variable interest rate basis spread (as a percent)			0.25%
Minimum [Member] \| Revolving Credit Facility [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Credit facility variable interest rate basis spread (as a percent)			1.25%
Maximum [Member] \| Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility commitment fee per annum on daily unused portion (as a percent)			0.30%
Maximum [Member] \| Revolving Credit Facility [Member] \| Base Rate [Member]
Line of Credit Facility [Line Items]
Credit facility variable interest rate basis spread (as a percent)			1.00%
Maximum [Member] \| Revolving Credit Facility [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Credit facility variable interest rate basis spread (as a percent)			2.00%
Weighted Average [Member] \| Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility interest rate during the period				2.58%
Subsequent Event [Member] \| Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility principal payment	$ 10,000
Credit facility remaining borrowing capacity							$ 280,000

Shareholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands					12 Months Ended
	Jan. 08, 2018 $ / shares	Dec. 12, 2017 $ / shares	Jun. 16, 2017 $ / shares	May 24, 2017 $ / shares	Dec. 31, 2017 USD ($) right / shares $ / right $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted \| $ / shares					$ 16.78	$ 12.43	$ 12.16
Increase in stock-based compensation from change in management estimate \| $					$ 11,206
Unvested employee stock options outstanding \| shares					6,088,000
Unrecognized compensation cost from employee stock options \| $					$ 62,070
Weighted average period for recognition of unrecognized compensation cost					2 years 3 months 18 days
Total options that were exercisable \| shares					10,624,000	6,692,000	8,508,000
Options outstanding weighted average remaining contractual life, years					6 years 3 months 18 days
Total intrinsic value of options exercised \| $					$ 83,628	$ 41,607
Aggregate intrinsic value of options exercisable \| $					455,528
Total intrinsic value of options outstanding \| $					$ 572,024
Share price of SEI common stock \| $ / shares					$ 71.86
Rights received by common shareholder for each common share outstanding, rights \| right / shares					1
Series A Junior Participating Preferred stock, par value \| $ / shares					$ 0.05	$ 0.05
Securities and assets of equivalent value, purchase price, per unit \| $ / shares					$ 150.00
Number of days, rights become exercisable following public announcement					10 days
Acquiring person, beneficial ownership of the outstanding common stock, minimum percentage					20.00%
Common shares received by holders upon surrender of rights, percentage					50.00%
Rights non-voting, redeemable days					10 days
Right redeemable price, per right \| $ / right					0.01
Dividends declared per common share \| $ / shares		$ 0.30		$ 0.28	$ 0.58	$ 0.54	$ 0.50
Dividends paid per common share \| $ / shares			$ 0.28
Dividends declared \| $					$ 91,444	$ 86,657	$ 82,478
Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividends paid per common share \| $ / shares	$ 0.30
Common Stock Buyback [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining stock repurchase authorization amount \| $					$ 170,638
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration date for options outstanding					Dec. 16, 2018
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration date for options outstanding					Dec. 12, 2027
2014 Equity Compensation Plan [Member] \| Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate					50.00%
2014 Equity Compensation Plan [Member] \| Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate					50.00%
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee stock purchase plan, employee eligible percentage on offerings of common stock					85.00%
Shares reserved under the plan \| shares					15,600,000
Issuance of common stock under the employee stock purchase plan, shares \| shares					11,962,000

Shareholders' Equity (Assumptions Used In The Weighted Average Fair Value Of Stock Options Granted) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected term	6 years	6 years	5 years 6 months 29 days
Expected volatility	22.58%	25.44%	23.86%
Expected dividend yield	0.82%	1.10%	1.00%
Risk-free interest rate	2.29%	2.18%	1.90%

Shareholders' Equity (Stock-Based Compensation Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' Equity Note [Abstract]
Stock-based compensation expense	$ 36,366	$ 16,017	$ 17,312
Less: Deferred tax benefit	(7,891)	(5,612)	(6,107)
Stock-based compensation expense, net of tax	$ 28,475	$ 10,405	$ 11,205

Shareholders' Equity (Stock Option Plans) (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance	18,069	19,237	20,461
Balance, weighted avg. price	$ 31.57	$ 28.71	$ 25.26
Granted	2,057	2,310	2,005
Granted, weighted avg. price	$ 69.87	$ 49.57	$ 53.34
Exercised	(2,370)	(1,809)	(2,927)
Exercised, weighted avg. price	$ 21.22	$ 24.82	$ 21.44
Expired or canceled	(1,044)	(1,669)	(302)
Expired or canceled, weighted avg. price	$ 33.42	$ 30.86	$ 28.97
Balance	16,712	18,069	19,237
Balance, weighted avg. price	$ 37.63	$ 31.57	$ 28.71
Total options that were exercisable	10,624	6,692	8,508
Exercisable shares, weighted avg. price	$ 28.99
Available for future grant as of December 31, 2017	24,108

Shareholders' Equity (Stock Options Outstanding And Exercisable) (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number of Shares	16,712	18,069	19,237	20,461
Weighted Average Exercise Price (Per Share)	$ 37.63	$ 31.57	$ 28.71	$ 25.26
Weighted Average Remaining Contractual Life (Years)	6 years 3 months 18 days
Number of Shares	10,624	6,692	8,508
Weighted Average Exercise Price (Per Share)	$ 28.99
Exercise Price $14.62-15.77 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range	14.62
Upper range	$ 15.77
Number of Shares	2,178
Weighted Average Exercise Price (Per Share)	$ 15.45
Weighted Average Remaining Contractual Life (Years)	3 years 1 month 28 days
Number of Shares	2,178
Weighted Average Exercise Price (Per Share)	$ 15.45
Weighted Average Remaining Contractual Life (Years)	3 years 1 month 28 days
Exercise Price $17.65-21.05 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range	$ 17.65
Upper range	$ 21.05
Number of Shares	1,174
Weighted Average Exercise Price (Per Share)	$ 17.67
Weighted Average Remaining Contractual Life (Years)	2 years 3 days
Number of Shares	1,174
Weighted Average Exercise Price (Per Share)	$ 17.67
Weighted Average Remaining Contractual Life (Years)	2 years 3 days
Exercise Price $22.45-23.86 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range	$ 22.45
Upper range	$ 23.86
Number of Shares	3,432
Weighted Average Exercise Price (Per Share)	$ 23.19
Weighted Average Remaining Contractual Life (Years)	3 years 11 months 12 days
Number of Shares	3,422
Weighted Average Exercise Price (Per Share)	$ 23.19
Weighted Average Remaining Contractual Life (Years)	3 years 11 months 15 days
Exercise Price $27.03-39.15 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range	$ 27.03
Upper range	$ 39.15
Number of Shares	1,815
Weighted Average Exercise Price (Per Share)	$ 33.65
Weighted Average Remaining Contractual Life (Years)	6 years 1 month 2 days
Number of Shares	747
Weighted Average Exercise Price (Per Share)	$ 33.58
Weighted Average Remaining Contractual Life (Years)	6 years 1 month 2 days
Exercise Price $40.64-53.16 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range	$ 40.64
Upper range	$ 53.16
Number of Shares	4,190
Weighted Average Exercise Price (Per Share)	$ 45.49
Weighted Average Remaining Contractual Life (Years)	8 years 6 months 14 days
Number of Shares	2,095
Weighted Average Exercise Price (Per Share)	$ 45.49
Weighted Average Remaining Contractual Life (Years)	8 years 29 days
Exercise Price $53.34-71.12 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Lower range	$ 53.34
Upper range	$ 71.12
Number of Shares	3,923
Weighted Average Exercise Price (Per Share)	$ 62.01
Weighted Average Remaining Contractual Life (Years)	9 years 18 days
Number of Shares	1,008
Weighted Average Exercise Price (Per Share)	$ 53.39
Weighted Average Remaining Contractual Life (Years)	8 years 1 month 20 days

Shareholders' Equity (Common Stock Buyback) (Details) - Common Stock Buyback [Member] - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Shares repurchased and retired during period	4,403	6,600	5,951
Value of shares repurchased during period	$ 248,114	$ 294,374	$ 289,587

Accumulated Other Comprehensive Income (Loss), Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ (37,955)	$ (25,290)	$ (8,895)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Other comprehensive gain (loss) before reclassifications	17,787	(13,049)	(16,559)
Amounts reclassified from accumulated other comprehensive loss	260	384	164
Net current-period other comprehensive gain (loss)	18,047	(12,665)	(16,395)
Ending Balance	(19,908)	(37,955)	(25,290)
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(37,119)	(24,988)	(10,088)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Other comprehensive gain (loss) before reclassifications	17,597	(12,131)	(14,900)
Amounts reclassified from accumulated other comprehensive loss	0	0	0
Net current-period other comprehensive gain (loss)	17,597	(12,131)	(14,900)
Ending Balance	(19,522)	(37,119)	(24,988)
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(836)	(302)	1,193
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Other comprehensive gain (loss) before reclassifications	190	(918)	(1,659)
Amounts reclassified from accumulated other comprehensive loss	260	384	164
Net current-period other comprehensive gain (loss)	450	(534)	(1,495)
Ending Balance	$ (386)	$ (836)	$ (302)

Commitments and Contingencies (Schedule Of Noncancellable Minimum Commitments) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Future minimum operating lease payments due in 2018	$ 10,046
Future minimum operating lease payments due in 2019	9,427
Future minimum operating lease payments due in 2020	7,413
Future minimum operating lease payments due in 2021	7,082
Future minimum operating lease payments due in 2022 and thereafter	29,171
Operating leases, future minimum payments due	$ 63,139

Income Taxes (Schedule Of Federal And State And Foreign Income Tax Provision) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal, current	$ 154,776	$ 158,411	$ 159,774
State, current	11,645	10,500	7,756
Foreign, current	8,002	5,137	5,224
Current federal, state and foreign income tax	174,423	174,048	172,754
Federal, deferred	(26,350)	788	(5,343)
State, deferred	1,378	(168)	1,414
Foreign, deferred	3,199	0	0
Deferred income tax expense (benefit)	(21,773)	620	(3,929)
Total income taxes	$ 152,650	$ 174,668	$ 168,825

Income Taxes (Components Of Net Income Before Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Domestic	$ 523,044	$ 481,760	$ 472,384
Foreign	33,995	26,725	28,096
Income before income taxes	$ 557,039	$ 508,485	$ 500,480

Income Taxes (Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate) (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory rate									35.00%	35.00%	35.00%
State taxes, net of federal tax benefit									1.30%	1.30%	1.60%
Foreign tax expense and tax rate differential									(1.10%)	(0.80%)	(1.20%)
Tax benefit from stock option exercises									(3.90%)	0.00%	0.00%
Change in enacted tax rate									(4.90%)	0.00%	0.00%
Repatriation of foreign earnings									2.60%	0.00%	0.00%
Research and development tax credit									(0.90%)	(0.80%)	(0.60%)
Domestic Production Activities Deduction									(0.50%)	(0.60%)	(0.60%)
Settlement of state tax petition									0.00%	0.00%	(0.80%)
Other, net									(0.20%)	0.20%	0.30%
Effective income tax rate	19.90%	27.70%	32.10%	31.00%	33.40%	33.80%	35.20%	35.20%	27.40%	34.30%	33.70%

Income Taxes (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Statutory rate			35.00%	35.00%	35.00%
Estimated liability for the one-time transition tax under the Tax Act	$ 11,544		$ 11,544
Estimated liability for the one-time transition tax under the Tax Act, short term	915		915
Estimated liability for the one-time transition tax under the Tax Act, long term	10,629		10,629
Foreign withholding tax included in Deferred income taxes	3,199		3,199	$ 0
Tax benefit from adjustment of estimated deferred tax liability			27,153
Tax expense from preliminary estimate of one-time transition tax and withholding tax			14,743
Net tax benefit from enactment of the Tax Act	$ 12,410
Net tax benefit from enactment of the Tax Act, per diluted share	$ 0.08
Unrecognized tax benefit, excluding interest and penalties	$ 14,480		14,480	17,287	$ 14,517	$ 14,018
Unrecognized tax benefits that would affect effective tax rate	13,737		13,737
Amount of previously unrecognized tax benefits recognized			5,846
Liabilities for tax-related interest and penalties			1,175	$ 1,227	$ 1,391
Current liabilities [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Amount of uncertain tax liability expected to be paid within one year	3,275		3,275
Other long-term liabilities [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Amount of uncertain tax liability included in Other long-term liabilities	12,380		12,380
Settlement and Lapse of Statute [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Unrecognized tax benefits within the next 12 months	$ 3,275		$ 3,275
Subsequent Event [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Statutory rate		21.00%

Income Taxes Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Deferred Tax Assets And Liabilities [Abstract]
Stock-based compensation expense	$ 24,725	$ 33,459
Foreign and state net operating loss carryforward	71,236	20,049
Basis differences in investments	4,191	6,165
Federal benefit of state tax deduction for uncertain tax positions	1,918	3,647
Revenue and expense recognized in different periods for financial reporting and income tax purposes	2,631	5,022
Other assets	273	813
Total deferred income tax assets	104,974	69,155
Less: valuation allowance	(68,469)	(17,922)
Net deferred income tax assets	36,505	51,233
Capitalized software currently deductible for tax purposes, net of amortization	(70,575)	(107,897)
Difference in financial reporting and income tax depreciation methods	(3,182)	(5,190)
Difference between book and tax basis of other assets	(3,549)	(4,597)
Goodwill and other intangibles	(1,001)	0
Foreign Dividend Withholding Tax	(3,199)	0
Other liabilities	(704)	(1,115)
Total deferred income tax liabilities	(82,210)	(118,799)
Net deferred income tax liabilities	$ (45,705)	$ (67,566)

Income Taxes (Unrecognized Tax Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 17,287	$ 14,517	$ 14,018
Gross additions, current year	3,180	3,756	1,954
Gross additions, prior year	211	1,762	297
Settlements	(352)	(378)	0
Lapses on statute of limitations	(5,846)	(2,370)	(1,752)
Ending balance	$ 14,480	$ 17,287	$ 14,517

Business Segment Information (Schedule Of Financial Information About Business Segments) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 408,219	$ 386,018	$ 372,331	$ 359,984	$ 368,810	$ 354,641	$ 343,831	$ 334,263	$ 1,526,552	$ 1,401,545	$ 1,334,208
Expenses									1,065,774	966,534	921,544
Operating profit (loss)									460,778	435,011	412,664
Gain on sale of subsidiary									0	2,791	2,791
Total profit (loss)									460,778	437,802	415,455
Private Banks [Member]
Segment Reporting Information [Line Items]
Revenues									474,272	457,886	456,516
Expenses									455,119	421,188	410,975
Operating profit (loss)									19,153	36,698	45,541
Gain on sale of subsidiary									0	2,791	2,791
Total profit (loss)									19,153	39,489	48,332
Investment Advisors [Member]
Segment Reporting Information [Line Items]
Revenues									373,473	330,677	306,620
Expenses									201,833	180,140	171,968
Operating profit (loss)									171,640	150,537	134,652
Gain on sale of subsidiary									0	0	0
Total profit (loss)									171,640	150,537	134,652
Institutional Investors [Member]
Segment Reporting Information [Line Items]
Revenues									322,457	312,584	297,568
Expenses									161,640	153,117	145,851
Operating profit (loss)									160,817	159,467	151,717
Gain on sale of subsidiary									0	0	0
Total profit (loss)									160,817	159,467	151,717
Investment Managers [Member]
Segment Reporting Information [Line Items]
Revenues									349,444	294,390	267,963
Expenses									226,504	191,127	172,094
Operating profit (loss)									122,940	103,263	95,869
Gain on sale of subsidiary									0	0	0
Total profit (loss)									122,940	103,263	95,869
Investments In New Businesses [Member]
Segment Reporting Information [Line Items]
Revenues									6,906	6,008	5,541
Expenses									20,678	20,962	20,656
Operating profit (loss)									(13,772)	(14,954)	(15,115)
Gain on sale of subsidiary									0	0	0
Total profit (loss)									$ (13,772)	$ (14,954)	$ (15,115)

Business Segment Information (Reconciliation Of Total Operating Profit Reported For Business Segments To Income From Operations In Consolidated Statements Of Operations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total operating profit from segments above	$ 460,778	$ 435,011	$ 412,664
Income from operations	396,944	375,694	358,213
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total operating profit from segments above	460,778	435,011	412,664
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Corporate overhead expenses	$ (63,834)	$ (59,317)	$ (54,451)

Business Segment Information (Schedule Of Additional Information Pertaining To Business Segments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Capital Expenditures	$ 86,568	$ 81,789	$ 73,881
Depreciation	27,311	26,440	24,044
Amortization	48,275	45,392	42,630
Assets	1,853,369	1,636,823
Private Banks [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	47,526	45,940	41,972
Depreciation	16,479	13,222	12,348
Amortization	32,696	31,675	29,819
Assets	523,214	471,435
Investment Advisors [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	17,450	17,610	13,206
Depreciation	3,364	3,880	3,410
Amortization	10,930	10,458	9,880
Assets	139,697	160,717
Institutional Investors [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	4,020	4,319	5,301
Depreciation	1,121	1,367	1,200
Amortization	1,599	1,674	1,558
Assets	117,286	109,879
Investment Managers [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	15,863	11,209	10,119
Depreciation	4,698	4,877	4,040
Amortization	2,593	1,092	1,029
Assets	278,540	158,621
Investments In New Businesses [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	546	726	736
Depreciation	826	2,197	2,278
Amortization	239	146	116
Assets	19,269	4,910
Total From Business Segments [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	85,405	79,804	71,334
Depreciation	26,488	25,543	23,276
Amortization	48,057	45,045	42,402
Assets	1,078,006	905,562
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Capital Expenditures	1,163	1,985	2,547
Depreciation	823	897	768
Amortization	218	347	$ 228
Assets	$ 775,363	$ 731,261

Business Segment Information (Schedule of Revenues And Assets Based On Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues	$ 408,219	$ 386,018	$ 372,331	$ 359,984	$ 368,810	$ 354,641	$ 343,831	$ 334,263	$ 1,526,552	$ 1,401,545	$ 1,334,208
Assets	1,853,369				1,636,823				1,853,369	1,636,823
United States [Member]
Revenues									1,298,381	1,191,640	1,123,165
Assets	1,471,260				1,317,235				1,471,260	1,317,235
International Operations [Member]
Revenues									228,171	209,905	$ 211,043
Assets	$ 382,109				$ 319,588				$ 382,109	$ 319,588

Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset management, administration and distribution fees [Member]
Related Party Transaction [Line Items]
Revenue from related parties	$ 453,438	$ 431,318	$ 426,301
Transaction-based and trade execution fees [Member]
Related Party Transaction [Line Items]
Revenue from related parties	$ 1,216	$ 561	$ 365

Sale of SEI Asset Korea (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 28, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of subsidiary		$ 0	$ 2,791	$ 2,791
SEI Asset Korea Co., Ltd. [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contingent purchase price from sale of SEI AK	$ 11,220
Gain on sale of subsidiary			$ 2,791	$ 2,791
Gain on sale of subsidiary, diluted earnings per share impact, net			$ 0.01	$ 0.01

- Definition

The amount of net income (loss) for the period attributable to extraordinary items that is available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(14))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e1278-109256

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(16))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e1311-109256

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(18))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Business Acquisitions Business Acquisitions (Details) - USD ($) $ in Thousands		12 Months Ended
	Jul. 03, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Net cash paid to acquire Archway		$ 80,234	$ 0	$ 0
Net assets acquired based on fair values:
Property and equipment	$ 776
Goodwill		52,990	0
Amortization of intangible assets		48,275	$ 45,392	$ 42,630
Estimated amortization expense related to Archway intangible assets in 2018		43,717
Estimated amortization expense related to Archway intangible assets in 2019		43,717
Estimated amortization expense related to Archway intangible assets in 2020		43,717
Estimated amortization expense related to Archway intangible assets in 2021		43,439
Estimated amortization expense related to Archway intangible assets in 2022		29,556
Archway [Member]
Business Acquisition [Line Items]
Total purchase price	81,635
Maximum contingent purchase price	8,000
Contingent consideration	4,800
Net adjustment to contingent consideration		3,800
Cash acquired during transaction	1,401
Net cash paid to acquire Archway	80,234
Escrow account balance		8,000
Net assets acquired based on fair values:
Current assets, net	2,539
Goodwill	52,990
Contingent consideration	(4,800)
Total purchase price allocation	81,635
Amortization of intangible assets		1,552
Estimated amortization expense related to Archway intangible assets in 2018		3,104
Estimated amortization expense related to Archway intangible assets in 2019		3,104
Estimated amortization expense related to Archway intangible assets in 2020		3,104
Estimated amortization expense related to Archway intangible assets in 2021		3,104
Estimated amortization expense related to Archway intangible assets in 2022		$ 2,757
Acquired technology [Member] \| Archway [Member]
Net assets acquired based on fair values:
Intangible assets	$ 13,510
Weighted average useful life of acquired intangible assets	10 years
Client relationships [Member] \| Archway [Member]
Net assets acquired based on fair values:
Intangible assets	$ 10,760
Weighted average useful life of acquired intangible assets	15 years
Non-competition agreements [Member] \| Archway [Member]
Net assets acquired based on fair values:
Intangible assets	$ 3,470
Weighted average useful life of acquired intangible assets	5 years
Trade names [Member] \| Archway [Member]
Net assets acquired based on fair values:
Intangible assets	$ 2,390
Weighted average useful life of acquired intangible assets	7 years

Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Revenues	$ 408,219	$ 386,018	$ 372,331	$ 359,984	$ 368,810	$ 354,641	$ 343,831	$ 334,263	$ 1,526,552	$ 1,401,545	$ 1,334,208
Income before income taxes	152,452	140,769	135,158	128,660	133,053	130,890	124,904	119,638	$ 557,039	$ 508,485	$ 500,480
Net income	$ 122,144	$ 101,739	$ 91,769	$ 88,737	$ 88,611	$ 86,704	$ 81,005	$ 77,497
Basic earnings per common share	$ 0.78	$ 0.64	$ 0.58	$ 0.56	$ 0.55	$ 0.54	$ 0.50	$ 0.48	$ 2.56	$ 2.07	$ 2.00
Diluted earnings per common share	$ 0.75	$ 0.63	$ 0.57	$ 0.55	$ 0.55	$ 0.53	$ 0.49	$ 0.47	$ 2.49	$ 2.03	$ 1.96
Effective income tax rate	19.90%	27.70%	32.10%	31.00%	33.40%	33.80%	35.20%	35.20%	27.40%	34.30%	33.70%
Net tax benefit from enactment of the Tax Act	$ 12,410
Net tax benefit from enactment of the Tax Act, per diluted share	$ 0.08
Extraordinary Item [Member]
Extraordinary Item [Line Items]
Performance fee associated with SEI-sponsored investment product					$ 12,286
Sub-advisory expense associated with SEI-sponsored investment product					6,143
Profit from extraordinary item, net of tax					$ 4,091
Profit from extraordinary item, net of tax, per diluted share					$ 0.03

Schedule II - Valuation And Qualifying Accounts And Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance For Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 523	$ 649	$ 784
Additions, Charged to Costs and Expenses	172	0	0
Additions, Charged to Other Accounts	0	0	0
(Deductions)	0	(126)	(135)
Balance at End of Year	695	523	649
Deferred Income Tax Valuation Allowance [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	17,922	14,548	16,509
Additions, Charged to Costs and Expenses	0	0	(1,142)
Additions, Charged to Other Accounts	50,547	3,374	(819)
(Deductions)	0	0	0
Balance at End of Year	$ 68,469	$ 17,922	$ 14,548

Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2017
Retirement Benefits [Abstract]
Employee Benefit Plan	Employee Benefit PlanThe Company has a tax-qualified defined contribution plan (the Plan). The Plan provides retirement benefits, including provisions for early retirement and disability benefits, as well as a tax-deferred savings feature. After satisfying certain requirements, participants are vested in employer contributions at the time the contributions are made. All Company contributions are discretionary and are made from available profits. The Company contributed $10,929, $9,665 and $9,162 to the Plan in 2017, 2016 and 2015, respectively.

Employee Benefit Plan (Details) - USD ($) $ in Thousands	12 Months Ended
Employee Benefit Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Defined contribution plan, cost recognized	$ 10,929	$ 9,665	$ 9,162