Summary of Significant Accounting Policies (Policy)	3 Months Ended
Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	The accompanying Consolidated Financial Statements have been prepared in accordance with accounting principles generally accepted in the United States of America.
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 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 $323,941 and $347,082 at March 31, 2021 and December 31, 2020, 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 March 31, 2021 and December 31, 2020 segregated for regulatory purposes related to trade-execution services conducted by SEI Investments (Europe) Limited. Restricted cash also includes $101 at March 31, 2021 and December 31, 2020 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.
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 three months ended March 31, 2021 were consistent with those as described in the Company's Annual Report on Form 10-K at December 31, 2020. The Company had no Level 3 financial assets at March 31, 2021 or December 31, 2020 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)

3 Months Ended

Mar. 31, 2021

The accompanying Consolidated Financial Statements have been prepared in accordance with accounting principles generally accepted in the United States of America.

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 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 $323,941 and $347,082 at March 31, 2021 and December 31, 2020, 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 March 31, 2021 and December 31, 2020 segregated for regulatory purposes related to trade-execution services conducted by SEI Investments (Europe) Limited. Restricted cash also includes $101 at March 31, 2021 and December 31, 2020 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.

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 three months ended March 31, 2021 were consistent with those as described in the Company's Annual Report on Form 10-K at December 31, 2020. The Company had no Level 3 financial assets at March 31, 2021 or December 31, 2020 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.