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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, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 793,883 |  | $ 793,883 |  | $ 784,626
Restricted cash | 351 |  | 351 |  | 3,101
Capitalized software development costs |  |  | 19,486 | $ 18,640 |
Net book value of capitalized software | $ 250,280 |  | $ 250,280 |  | 270,977
Anti-dilutive employee stock options (in shares) | 11,347 | 8,813 | 11,531 | 8,342 |
Anti-dilutive employee stock options (in USD per share) | $ 57.80 | $ 57.98 | $ 57.78 | $ 58.18 |
SEI Wealth Platform |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | $ 19,387 | $ 17,208 |
Net book value of capitalized software | $ 241,262 |  | 241,262 |  |
Capitalized software in progress | 22,702 |  | 22,702 |  | 13,409
Amortization expense of capitalized software |  |  | $ 35,834 | 32,576 |
SEI Wealth Platform | Weighted Average |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Weighted average remaining life (in years) |  |  | 10 years |  |
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 | 332,657 |  | $ 332,657 |  | 347,082
SEI Investments (Europe) Limited |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Restricted cash | 250 |  | 250 |  | 3,000
SEI Investments Distribution Co. (SIDCO) |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Restricted cash | 101 |  | 101 |  | $ 101
SEI-Sponsored Open-Ended Money Market Mutual Funds |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Fees waived | $ 11,629 | $ 8,575 | $ 33,118 | $ 24,930 |

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