Summary of Significant Accounting Policies (Policy)	6 Months Ended
Jun. 30, 2020
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 June 30, 2020, the results of operations for the three and six months ended June 30, 2020 and 2019, and cash flows for the six-months ended June 30, 2020 and 2019. 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, 2019. The Company adopted the requirements of Accounting Standards Update (ASU) 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (Accounting Standards Codification (ASC) 326)) (ASU 2016-13) and subsequent amendments ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments-Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments (ASU 2019-04) and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments-Credit Losses (ASU 2019-11) on January 1, 2020. ASU 2016-13 and the related amendments are hereafter referred to as ASC 326. ASC 326 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. The Company owns mortgage-backed securities issued by the Government National Mortgage Association (GNMA), a federal agency of the U.S. government classified as Available-for-sale debt securities which qualify for the zero credit risk allowance. The Company's U.S. Treasury and other U.S. government agency securities classified as Securities owned are outside the scope of ASC 326. There was no impact to the Company's disclosures related to its marketable securities from the implementation of ASC 326. In accordance with ASC 326, the Company evaluated its receivable balances for credit risk based upon the source of revenue, its ability to collect fees directly from investment products or directly from assets in the client's account, a review of actual historical credit losses, and the potential for expected credit loss from its current client base. The Company has no meaningful historical credit loss data and a very limited amount of losses pertaining directly to a client's inability to satisfy its receivable balance even during periods of economic distress. The credit loss reserve recognized by the Company through the implementation of ASC 326 during the six months ended June 30, 2020 was immaterial. The Company also adopted the requirements of ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASU 2017-04) on January 1, 2020. The adoption of ASU 2017-04 did not have a material impact on the Company's consolidated financial statements and related disclosures. With the exception of the adoption of ASC 326 and ASU 2017-04, there have been no other significant changes in significant accounting policies during the six months ended June 30, 2020 as compared to the significant accounting policies described in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.
Variable Interest Entities	Variable Interest Entities 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 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.
Revenue Recognition	Revenue RecognitionRevenue is recognized when the transfer of control of promised goods or services under the terms of a contract with customers are satisfied in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those promised goods or services. Certain portions of the Company’s revenues involve a third party in providing goods or services to its customers. In such circumstances, the Company must determine whether the nature of its promise to the customer is to provide the underlying goods or services (the Company 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).
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents includes $335,676 and $414,581 at June 30, 2020 and December 31, 2019, respectively, primarily invested in SEI-sponsored open-ended money market mutual funds. 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.
Restricted Cash	Restricted CashRestricted cash includes $3,000 at June 30, 2020 and December 31, 2019 segregated for regulatory purposes related to trade-execution services conducted by SEI Investments (Europe) Limited. Restricted cash also includes $101 at June 30, 2020 and December 31, 2019 segregated in special reserve accounts for the benefit of customers of the Company’s broker-dealer subsidiary, SEI Investments Distribution Co. (SIDCO), in accordance with certain rules established by the Securities and Exchange Commission (SEC) for broker-dealers.
New Accounting Pronouncements	New Accounting Pronouncements In December 2019, the Financial Accounting Standards Board (FASB) issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (ASU 2019-12). The standard removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. ASU 2019-12 is effective for the Company beginning in the first quarter of 2021. The Company is currently evaluating the impact of adopting ASU 2019-12 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% 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.
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 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 2023 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 for our Level 1 and Level 2 financial assets during the six months ended June 30, 2020 were consistent with those as described in our Annual Report on Form 10-K at December 31, 2019. The Company had no Level 3 financial assets at June 30, 2020 or December 31, 2019 that were required to be measured at fair value on a recurring basis.
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 gains 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.

Summary of Significant Accounting Policies (Policy)

6 Months Ended

Jun. 30, 2020

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 June 30, 2020, the results of operations for the three and six months ended June 30, 2020 and 2019, and cash flows for the six-months ended June 30, 2020 and 2019. 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, 2019.

The Company adopted the requirements of Accounting Standards Update (ASU) 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (Accounting Standards Codification (ASC) 326)) (ASU 2016-13) and subsequent amendments ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments-Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments (ASU 2019-04) and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments-Credit Losses (ASU 2019-11) on January 1, 2020. ASU 2016-13 and the related amendments are hereafter referred to as ASC 326. ASC 326 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. The Company owns mortgage-backed securities issued by the Government National Mortgage Association (GNMA), a federal agency of the U.S. government classified as Available-for-sale debt securities which qualify for the zero credit risk allowance. The Company's U.S. Treasury and other U.S. government agency securities classified as Securities owned are outside the scope of ASC 326. There was no impact to the Company's disclosures related to its marketable securities from the implementation of ASC 326.

In accordance with ASC 326, the Company evaluated its receivable balances for credit risk based upon the source of revenue, its ability to collect fees directly from investment products or directly from assets in the client's account, a review of actual historical credit losses, and the potential for expected credit loss from its current client base. The Company has no meaningful historical credit loss data and a very limited amount of losses pertaining directly to a client's inability to satisfy its receivable balance even during periods of economic distress. The credit loss reserve recognized by the Company through the implementation of ASC 326 during the six months ended June 30, 2020 was immaterial.

The Company also adopted the requirements of ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASU 2017-04) on January 1, 2020. The adoption of ASU 2017-04 did not have a material impact on the Company's consolidated financial statements and related disclosures.

With the exception of the adoption of ASC 326 and ASU 2017-04, there have been no other significant changes in significant accounting policies during the six months ended June 30, 2020 as compared to the significant accounting policies described in the Company's Annual Report on Form 10-K for the year ended December 31, 2019.

Variable Interest Entities

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 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.

Revenue Recognition

Revenue RecognitionRevenue is recognized when the transfer of control of promised goods or services under the terms of a contract with customers are satisfied in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those promised goods or services. Certain portions of the Company’s revenues involve a third party in providing goods or services to its customers. In such circumstances, the Company must determine whether the nature of its promise to the customer is to provide the underlying goods or services (the Company 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).

Cash and Cash Equivalents

Cash and cash equivalents includes $335,676 and $414,581 at June 30, 2020 and December 31, 2019, respectively, primarily invested in SEI-sponsored open-ended money market mutual funds.

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.

Restricted Cash

Restricted CashRestricted cash includes $3,000 at June 30, 2020 and December 31, 2019 segregated for regulatory purposes related to trade-execution services conducted by SEI Investments (Europe) Limited. Restricted cash also includes $101 at June 30, 2020 and December 31, 2019 segregated in special reserve accounts for the benefit of customers of the Company’s broker-dealer subsidiary, SEI Investments Distribution Co. (SIDCO), in accordance with certain rules established by the Securities and Exchange Commission (SEC) for broker-dealers.

New Accounting Pronouncements

In December 2019, the Financial Accounting Standards Board (FASB) issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (ASU 2019-12). The standard removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. ASU 2019-12 is effective for the Company beginning in the first quarter of 2021. The Company is currently evaluating the impact of adopting ASU 2019-12 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% 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.

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 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 2023 to 2041.

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 for our Level 1 and Level 2 financial assets during the six months ended June 30, 2020 were consistent with those as described in our Annual Report on Form 10-K at December 31, 2019. The Company had no Level 3 financial assets at June 30, 2020 or December 31, 2019 that were required to be measured at fair value on a recurring basis.

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 gains 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.