Summary of Significant Accounting Policies (Policy)	3 Months Ended
Mar. 31, 2015
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, 2015, the results of operations for the three months ended March 31, 2015 and 2014, and cash flows for the three month periods ended March 31, 2015 and 2014. 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, 2014. There have been no significant changes in significant accounting policies during the three months ended March 31, 2015 as compared to the significant accounting policies described in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014.
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 Financial Accounting Standards Board (FASB) issued Accounting Standards Update 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. Early adoption is not permitted. ASU 2014-09 currently becomes effective for the Company during the first quarter 2017. In April 2015, the FASB proposed a one year delay of the effective date of ASU 2014-09. 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. On February 18, 2015, the FASB issued Accounting Standards Update No. 2015-02, Consolidation (Topic 810) - Amendments to the Consolidation Analysis (ASU 2015-02). The new guidance applies to entities in all industries and provides a new scope exception to registered money market funds and similar unregistered money market funds. It makes targeted amendments to the current consolidation guidance and ends the deferral granted to investment companies from applying the VIE guidance. ASU 2015-02 becomes effective for the Company during the first quarter 2016. The Company is currently evaluating the effect that the standard will have on its consolidated financial statements and related disclosures.
Reclassifications	Reclassifications Certain prior year amounts have been reclassified to conform to current year presentation.
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.
Variable Interest Entities	The Company does not have a significant equity investment in any of the VIEs and does not have an obligation to enter into any guarantee agreements with the VIEs. The fees paid to the decision maker of a VIE are considered to be variable interests if the decision maker is not subject to substantive kick-out rights. The fees paid to the Company represent a variable interest when the decision maker is not subject to substantive kick-out rights. The Company is not the primary beneficiary of the VIEs because the expected fees and the expected return on any investment into the VIE by the Company relative to the expected returns of the VIE to the equity investor holders does not approach 50 percent of the expected losses or gains of the VIEs. Therefore, the Company is not required to consolidate any investment products that are VIEs into its financial statements. The Company’s variable interest in the VIEs, which consists of management fees and in some situations, seed capital, would not be considered a significant variable interest. The risks to the Company associated with its involvement with any of the investment products that are VIEs are limited to the cash flows received from the revenue generated for asset management, administration and distribution services and any equity investments in the VIEs. Both of these items are immaterial. The Company has no other financial obligation to the VIEs. Amounts relating to fees due from the VIEs included in Receivables and amounts relating to equity investments in the VIEs included in Investments Available for Sale on the Company’s Consolidated Balance Sheets are immaterial to the total current assets of the Company.
Fair Value of Financial Instruments	The fair value of the Company’s financial assets and liabilities 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 and investment grade commercial paper held by SIDCO, and investment funds sponsored by LSV. 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 2043. 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. As a practical expedient, the Company relies on the net asset values (NAVs) of the investment funds sponsored by LSV as the fair value. 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 pricing policies and procedures applied during the three months ended March 31, 2015 were consistent with those as described in our Annual Report on Form 10-K at December 31, 2014.
Available-for-sale Securities	These net unrealized gains are reported as a separate component of Accumulated other comprehensive loss on the accompanying Consolidated Balance Sheets.
Investments in Affiliated Funds	As a practical expedient, the Company relies on the net asset values (NAVs) of the investment funds sponsored by LSV as the fair value. 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.
Securities Owned	The Company’s broker-dealer subsidiary, SIDCO, has investments in U.S. government agency and commercial paper 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.

Summary of Significant Accounting Policies (Policy)

3 Months Ended

Mar. 31, 2015

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, 2015, the results of operations for the three months ended March 31, 2015 and 2014, and cash flows for the three month periods ended March 31, 2015 and 2014. 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, 2014.

There have been no significant changes in significant accounting policies during the three months ended March 31, 2015 as compared to the significant accounting policies described in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014.

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

On May 28, 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update 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. Early adoption is not permitted. ASU 2014-09 currently becomes effective for the Company during the first quarter 2017. In April 2015, the FASB proposed a one year delay of the effective date of ASU 2014-09. 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.

On February 18, 2015, the FASB issued Accounting Standards Update No. 2015-02, Consolidation (Topic 810) - Amendments to the Consolidation Analysis (ASU 2015-02). The new guidance applies to entities in all industries and provides a new scope exception to registered money market funds and similar unregistered money market funds. It makes targeted amendments to the current consolidation guidance and ends the deferral granted to investment companies from applying the VIE guidance. ASU 2015-02 becomes effective for the Company during the first quarter 2016. The Company is currently evaluating the effect that the standard will have on its consolidated financial statements and related disclosures.

Reclassifications

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

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.

Variable Interest Entities

The Company does not have a significant equity investment in any of the VIEs and does not have an obligation to enter into any guarantee agreements with the VIEs. The fees paid to the decision maker of a VIE are considered to be variable interests if the decision maker is not subject to substantive kick-out rights. The fees paid to the Company represent a variable interest when the decision maker is not subject to substantive kick-out rights.

The Company is not the primary beneficiary of the VIEs because the expected fees and the expected return on any investment into the VIE by the Company relative to the expected returns of the VIE to the equity investor holders does not approach 50 percent of the expected losses or gains of the VIEs. Therefore, the Company is not required to consolidate any investment products that are VIEs into its financial statements. The Company’s variable interest in the VIEs, which consists of management fees and in some situations, seed capital, would not be considered a significant variable interest.

The risks to the Company associated with its involvement with any of the investment products that are VIEs are limited to the cash flows received from the revenue generated for asset management, administration and distribution services and any equity investments in the VIEs. Both of these items are immaterial. The Company has no other financial obligation to the VIEs.

Amounts relating to fees due from the VIEs included in Receivables and amounts relating to equity investments in the VIEs included in Investments Available for Sale on the Company’s Consolidated Balance Sheets are immaterial to the total current assets of the Company.

Fair Value of Financial Instruments

The fair value of the Company’s financial assets and liabilities 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 and investment grade commercial paper held by SIDCO, and investment funds sponsored by LSV. 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 2043.

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. As a practical expedient, the Company relies on the net asset values (NAVs) of the investment funds sponsored by LSV as the fair value. 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 pricing policies and procedures applied during the three months ended March 31, 2015 were consistent with those as described in our Annual Report on Form 10-K at December 31, 2014.

Available-for-sale Securities

These net unrealized gains are reported as a separate component of Accumulated other comprehensive loss on the accompanying Consolidated Balance Sheets.

Investments in Affiliated Funds

As a practical expedient, the Company relies on the net asset values (NAVs) of the investment funds sponsored by LSV as the fair value. 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.

Securities Owned

The Company’s broker-dealer subsidiary, SIDCO, has investments in U.S. government agency and commercial paper 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.