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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023 | Dec. 31, 2023
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 768,291 |  | $ 768,291 |  | $ 834,697
Restricted cash | 301 |  | 301 |  | 301
Capitalized software development costs |  |  | 12,670 | $ 18,036 |
Net book value of capitalized software | $ 238,559 |  | $ 238,559 |  | 239,783
Anti-dilutive employee stock options (in shares) | 10,722 | 11,006 | 10,817 | 11,140 |
Anti-dilutive employee stock options (in USD per share) | $ 61.04 | $ 61.29 | $ 61.05 | $ 61.29 |
Investment Managers |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | $ 4,947 | $ 8,642 |
Capitalized software in progress | $ 25,115 |  | 25,115 |  | 20,083
SEI Wealth Platform |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | 7,723 | 9,394 |
Net book value of capitalized software | 212,558 |  | 212,558 |  |
Capitalized software in progress | $ 2,614 |  | 2,614 |  |
Amortization expense related to the SEI Wealth Platform |  |  | $ 13,599 | 12,516 |
SEI Wealth Platform | Weighted Average |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Estimated useful life of the SEI Wealth Platform | 8 years 7 months 6 days |  | 8 years 7 months 6 days |  |
SEI-Sponsored Open-Ended Money Market Mutual Funds |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 358,735 |  | $ 358,735 |  | 397,838
SEI Investments (Europe) Limited |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Restricted cash | 250 |  | 250 |  | 250
SEI Investments Distribution Co. (SIDCO) |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Restricted cash | 51 |  | 51 |  | $ 51
SEI-Sponsored Open-Ended Money Market Mutual Funds |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Fees waived | $ 1,263 | $ 5,320 | $ 5,562 | $ 10,881 |

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