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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, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Jan. 01, 2019 | Dec. 31, 2018
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Operating Lease Right-of-Use Assets | $ 41,054 |  | $ 41,054 |  |  |
Lease liabilities | 45,704 |  | 45,704 |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | 767,809 |  | 767,809 |  |  | $ 754,525
Restricted cash | 3,100 |  | 3,100 |  |  | 3,514
Capitalized software development costs |  |  | 26,821 | $ 33,371 |  |
Net book value of capitalized software | $ 300,848 |  | $ 300,848 |  |  | 309,500
Anti-dilutive employee stock options (in shares) | 6,239 | 6,183 | 6,269 | 6,153 |  |
Anti-dilutive employee stock options (in USD per share) | $ 54.91 | $ 53.38 | $ 54.84 | $ 53.15 |  |
SEI Wealth Platform |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Capitalized software development costs |  |  | $ 26,029 | $ 32,526 |  |
Net book value of capitalized software | $ 283,128 |  | 283,128 |  |  |
Capitalized software in progress | 51,781 |  | 51,781 |  |  | 42,238
Amortization expense of capitalized software |  |  | $ 31,567 | 29,723 |  |
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 | 319,977 |  | $ 319,977 |  |  | 315,840
SEI Investments (Europe) Limited |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Restricted cash | 3,000 |  | 3,000 |  |  | 3,000
SEI Investments Distribution Co. (SIDCO) |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Restricted cash | 100 |  | 100 |  |  | $ 514
SEI-Sponsored Open-Ended Money Market Mutual Funds |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Fees waived according to expense limitation agreements | $ 6,390 | $ 6,525 | $ 21,091 | $ 19,551 |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Operating Lease Right-of-Use Assets |  |  |  |  | $ 44,169 |
Lease liabilities |  |  |  |  | $ 44,169 |

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