Summary of Significant Accounting Policies (Policies)	6 Months Ended
Jun. 30, 2025
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, 2025, the results of operations for the three and six months ended June 30, 2025 and 2024, and cash flows for the six months ended June 30, 2025 and 2024. 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, 2024.
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 Recognition Revenue 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 The Company considers investment instruments purchased with an original maturity of three months or less to be cash equivalents.
Restricted Cash	cash of $250 at June 30, 2025 and December 31, 2024 segregated for regulatory purposes related to trade-execution services conducted by SEI Investments (Europe) Limited and $52 at June 30, 2025 and December 31, 2024 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 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvement to Income Tax Disclosures (ASU 2023-09) to enhance the transparency and decision usefulness of income tax disclosures. ASU 2023-09 is effective for annual periods beginning after December 15, 2024 on a prospective basis. Early adoption is permitted. The Company does not expect ASU 2023-09 to have a significant impact on its annual consolidated financial statements and related disclosures. In November 2024, the FASB issued ASU 2024-03, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses (ASU 2024-03) and in January 2025, the FASB issued ASU 2025-01, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Clarifying the Effective Date (ASU 2025-01), which clarified the effective date of ASU 2024-03. This standard requires new disclosures to disaggregate prescribed natural expenses underlying any income statement caption. ASU 2024-03 is effective for annual periods beginning after December 15, 2026 on a prospective basis and interim periods beginning after December 15, 2027. Early adoption is permitted. The Company is currently evaluating the impact of adopting the guidance 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’s interest in the net assets of LSV is included in Investments 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 is determined in accordance with the fair value hierarchy. The pricing policies and procedures applied to the Company's Level 1 and Level 2 financial assets during the six months ended June 30, 2025 were consistent with those as described in the Company's Annual Report on Form 10-K at December 31, 2024. The Company’s Level 1 financial assets consists mainly of investments in open-end and closed-end investment products 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), and 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 2027 to 2041. The Company's Level 3 financial liabilities at June 30, 2025 and December 31, 2024 consist entirely of the estimated fair value of the contingent considerations resulting from business acquisitions (See Note 12). The fair value of the contingent considerations were determined using a Monte-Carlo simulation model. There were no transfers of financial assets between levels within the fair value hierarchy during the six months ended June 30, 2025. The fair value of the Company's investments in 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 investment funds sponsored by LSV allow for investor redemptions at the end of each calendar month. The investments measured using the NAV as a practical expedient have not been classified in the fair value hierarchy but are presented in the tables below to permit reconciliation to the amounts presented on the accompanying Consolidated Balance Sheets.
Investments in Affiliated Funds	The fair value of the Company's investments in 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 investment funds sponsored by LSV allow for investor redemptions at the end of each calendar month. The investments measured using the NAV as a practical expedient have not been classified in the fair value hierarchy but are 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	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 (Policies)

6 Months Ended

Jun. 30, 2025

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, 2025, the results of operations for the three and six months ended June 30, 2025 and 2024, and cash flows for the six months ended June 30, 2025 and 2024. 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, 2024.

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 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

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

Restricted Cash

cash of $250 at June 30, 2025 and December 31, 2024 segregated for regulatory purposes related to trade-execution services conducted by SEI Investments (Europe) Limited and $52 at June 30, 2025 and December 31, 2024 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 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvement to Income Tax Disclosures (ASU 2023-09) to enhance the transparency and decision usefulness of income tax disclosures. ASU 2023-09 is effective for annual periods beginning after December 15, 2024 on a prospective basis. Early adoption is permitted. The Company does not expect ASU 2023-09 to have a significant impact on its annual consolidated financial statements and related disclosures.

In November 2024, the FASB issued ASU 2024-03, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses (ASU 2024-03) and in January 2025, the FASB issued ASU 2025-01, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Clarifying the Effective Date (ASU 2025-01), which clarified the effective date of ASU 2024-03. This standard requires new disclosures to disaggregate prescribed natural expenses underlying any income statement caption. ASU 2024-03 is effective for annual periods beginning after December 15, 2026 on a prospective basis and interim periods beginning after December 15, 2027. Early adoption is permitted. The Company is currently evaluating the impact of adopting the guidance 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’s interest in the net assets of LSV is included in Investments 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 is determined in accordance with the fair value hierarchy. The pricing policies and procedures applied to the Company's Level 1 and Level 2 financial assets during the six months ended June 30, 2025 were consistent with those as described in the Company's Annual Report on Form 10-K at December 31, 2024. The Company’s Level 1 financial assets consists mainly of investments in open-end and closed-end investment products 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), and 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 2027 to 2041. The Company's Level 3 financial liabilities at June 30, 2025 and December 31, 2024 consist entirely of the estimated fair value of the contingent considerations resulting from business acquisitions (See Note 12). The fair value of the contingent considerations were determined using a Monte-Carlo simulation model. There were no transfers of financial assets between levels within the fair value hierarchy during the six months ended June 30, 2025.

The fair value of the Company's investments in 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 investment funds sponsored by LSV allow for investor redemptions at the end of each calendar month. The investments measured using the NAV as a practical expedient have not been classified in the fair value hierarchy but are presented in the tables below to permit reconciliation to the amounts presented on the accompanying Consolidated Balance Sheets.

Investments in Affiliated Funds

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.