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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, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 780,928 |  | $ 780,928 |  | $ 784,626
Restricted cash | 351 |  | 351 |  | 3,101
Capitalized software development costs |  |  | 12,271 | $ 12,533 |
Net book value of capitalized software | $ 256,473 |  | $ 256,473 |  | 270,977
Anti-dilutive employee stock options (in shares) | 11,530 | 8,154 | 11,623 | 7,781 |
Anti-dilutive employee stock options (in USD per share) | $ 57.77 | $ 58.26 | $ 57.77 | $ 58.50 |
SEI Wealth Platform |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Capitalized software development costs |  |  | $ 12,172 | $ 11,785 |
Net book value of capitalized software | $ 246,005 |  | 246,005 |  |
Capitalized software in progress | 20,897 |  | 20,897 |  | 13,409
Amortization expense of capitalized software |  |  | $ 23,876 | 21,656 |
SEI Wealth Platform | Weighted Average |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Weighted average remaining life |  |  | 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 | 313,481 |  | $ 313,481 |  | 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 | $ 11,256 | $ 9,714 | $ 21,489 | $ 16,355 |

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