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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019
Summary of Significant Accounting Policies [Line Items] |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 746,870 |  | $ 841,446
Restricted cash | 3,101 |  | 3,101
Capitalized software development costs | 6,458 | $ 9,937 |
Net book value of capitalized software | $ 290,427 |  | 296,068
Anti-dilutive employee stock options (in shares) | 7,408 | 6,323 |
Anti-dilutive employee stock options (in USD per share) | $ 58.76 | $ 54.81 |
SEI Wealth Platform |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Capitalized software development costs | $ 6,229 | $ 9,739 |
Net book value of capitalized software | 274,885 |  |
Capitalized software in progress | 58,149 |  | 55,332
Amortization expense of capitalized software | $ 10,797 | 10,399 |
SEI Wealth Platform | Weighted Average |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Weighted Average Estimated Useful Life | 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 | $ 278,514 |  | 414,581
SEI Investments (Europe) Limited |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Restricted cash | 3,000 |  | $ 3,000
SEI Investments Distribution Co. (SIDCO) |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Restricted cash | 101 |  |
SEI-Sponsored Open-Ended Money Market Mutual Funds |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Fees waived | $ 6,641 | $ 7,905 |

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