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Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 901,134 |  | $ 901,134 |  | $ 834,697
Restricted cash | 302 |  | 302 |  | 301
Capitalized software development costs |  |  | 18,414 | $ 26,584 |
Net book value of capitalized software | $ 237,236 |  | $ 237,236 |  | 239,783
Anti-dilutive employee stock options (in shares) | 10,606 | 10,869 | 10,747 | 11,049 |
Anti-dilutive employee stock options (in USD per share) | $ 61.06 | $ 61.31 | $ 61.05 | $ 61.29 |
Investment Managers |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | $ 7,872 | $ 12,878 |
Capitalized software in progress | $ 28,040 |  | 28,040 |  |
SEI Wealth Platform |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | 10,542 | 13,706 |
Net book value of capitalized software | 208,457 |  | 208,457 |  |
Capitalized software in progress | $ 1,145 |  | 1,145 |  |
Amortization expense related to the SEI Wealth Platform |  |  | $ 20,517 | 19,027 |
SEI Wealth Platform | Weighted Average |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Estimated useful life of the SEI Wealth Platform | 8 years 4 months 24 days |  | 8 years 4 months 24 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 | $ 423,471 |  | $ 423,471 |  | 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 | 52 |  | 52 |  | $ 51
SEI-Sponsored Open-Ended Money Market Mutual Funds |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Fees waived | $ 1,202 | $ 5,519 | $ 6,764 | $ 16,400 |

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