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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, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | $ 22,828 | $ 18,414 |
Net book value of capitalized software | $ 235,559 |  | $ 235,559 |  | $ 236,023
Anti-dilutive employee stock options (in shares) | 5,687 | 10,606 | 5,744 | 10,747 |
Anti-dilutive employee stock options (in USD per share) | $ 66.09 | $ 61.06 | $ 66.04 | $ 61.05 |
Investment Managers |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | $ 8,318 | $ 7,872 |
Capitalized software in progress | $ 8,319 |  | 8,319 |  |
SEI Wealth Platform |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | 14,510 | 10,542 |
Net book value of capitalized software | 197,528 |  | 197,528 |  |
Capitalized software in progress | $ 1,862 |  | 1,862 |  |
Amortization expense related to the SEI Wealth Platform |  |  | $ 21,602 | 20,517 |
SEI Wealth Platform | Weighted Average |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Estimated useful life of the SEI Wealth Platform | 7 years 8 months 12 days |  | 7 years 8 months 12 days |  |
Investment Managers Platform |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Net book value of capitalized software | $ 38,031 |  | $ 38,031 |  |
Estimated useful life of the SEI Wealth Platform | 7 years |  | 7 years |  |
Amortization expense related to the SEI Wealth Platform |  |  | $ 1,100 |  |
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 | $ 191,938 |  | 191,938 |  | 341,311
SEI Investments (Europe) Limited |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents | 250 |  | 250 |  | 250
SEI Investments Distribution Co. (SIDCO) |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents | 52 |  | 52 |  | $ 52
SEI-Sponsored Open-Ended Money Market Mutual Funds |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Fees waived | $ 5,397 | $ 1,202 | $ 14,792 | $ 6,764 |

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