Summary of Significant Accounting Policies (Policy)	3 Months Ended
Summary of Significant Accounting Policies (Policy)	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying Consolidated Financial Statements have been prepared in accordance with accounting principles generally accepted in the United States of America. Certain financial information and accompanying note disclosure normally included in the Company’s Annual Report on Form 10-K have been condensed or omitted. The interim financial information is unaudited but reflects all adjustments (consisting of only normal recurring adjustments) which are, in the opinion of management, necessary for a fair statement of financial position of the Company as of March 31, 2017, the results of operations for the three months ended March 31, 2017 and 2016, and cash flows for the three-month periods ended March 31, 2017 and 2016. These interim Consolidated Financial Statements should be read in conjunction with the audited Consolidated Financial Statements and the Notes to the Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016. There have been no significant changes in significant accounting policies during the three months ended March 31, 2017 as compared to the significant accounting policies described in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016 with the exception of the adoption of Accounting Standards Update (ASU) 2016-09, Improvements to Employee Share-Based Payment Accounting (ASU 2016-09). 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 11). 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 the periods prior to the date of adoption. As a result, for the three months ended March 31, 2016, net cash provided by operating activities increased by $624 with a corresponding offset 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.
Reclassifications	Reclassifications Certain prior year amounts have been reclassified to conform to current year presentation.
Cash and Cash Equivalents	Statements of Cash Flows For purposes of the Consolidated Statements of Cash Flows, the Company considers investment instruments purchased with an original maturity of three months or less to be cash equivalents.
New Accounting Pronouncements	New Accounting Pronouncements On May 28, 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (ASU 2014-09), requiring an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. The updated standard permits the use of either the retrospective or cumulative effect transition method. The FASB has recently issued several amendments to the standard, including principal versus agent guidance and identifying performance obligations. ASU 2014-09 currently becomes effective for the Company during the first quarter 2018. The Company continues to assess the impact ASU 2014-09 will have on its revenue arrangements with completion, including selecting a transition method, expected by the third quarter of 2017. The majority of the Company’s services are bundled together and provided and completed for the client on a monthly basis. The Company expects revenue for these services to continue to be recognized monthly because of the continuous transfer of control to the client. Therefore, the adoption of this ASU is not expected to have a material impact on the recognition of revenue for the majority of the fees recognized for the services provided. However, the Company believes the adoption of the new standard may affect the timing of the recognition of implementation fees recognized in Information processing and software servicing fees as well as fund conversion fees and other ancillary fees recognized in Asset management, administration and distribution fees. Transaction-based and trade execution fees are based on current period activity and is not expected to be affected by the adoption of ASU 2014-09. Due to the complexity of some of our agreements, the actual revenue recognition treatment required under the standard will be dependent on contract-specific terms, and certain aspects may vary in some instances from recognition ratably over the contract term. The new standard also modified some of the principal and agent considerations which may result in changes to gross or net treatment of revenue and expenses but would not affect final net income. The Company is also evaluating its sales commission programs affected by the standard. Previously, sales commission costs were expensed at inception of a sales agreement but under the new standard will generally need to be capitalized and amortized over the period of contract performance. The new standard provides companies with alternative methods of adoption and the Company is in the process of determining the method of adoption, which depends in part upon completion of the evaluation of the remaining revenue arrangements. 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.
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 affiliate on the accompanying Consolidated Balance Sheets and its interest in the earnings of LSV is reflected in Equity in earnings of unconsolidated affiliate on the accompanying Consolidated Statements of Operations.
Variable Interest Entities	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 VOE model.
Fair Value of Financial Instruments	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 consist 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 pricing policies and procedures applied during the three months ended March 31, 2017 were consistent with those as described in our Annual Report on Form 10-K at December 31, 2016.
Investments in Affiliated Funds	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.
Available-for-sale Securities	These net unrealized losses are reported as a separate component of Accumulated other comprehensive loss on the accompanying Consolidated Balance Sheets.
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.