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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 684,971 | $ 623,162 | $ 684,971 | $ 623,162 | $ 695,701 | $ 679,661
Restricted cash | 3,503 |  | 3,503 |  | 3,500 |
Capitalized software development costs |  |  | 48,573 | $ 33,196 |  |
Net book value of capitalized software | $ 307,116 |  | $ 307,116 |  | 295,867 |
Anti-dilutive employee stock options | 11,324 | 10,258 | 11,286 | 10,384 |  |
Anti-dilutive employee stock options, per share | $ 37.81 | $ 34.11 | $ 37.73 | $ 34.07 |  |
SEI Wealth Platform [Member] |  |  |  |  |  |
Capitalized software development costs |  |  | $ 40,604 | $ 27,387 |  |
Net book value of capitalized software | $ 283,639 |  | 283,639 |  |  |
Capitalized software in progress | $ 27,344 |  | 27,344 |  |  |
Estimated useful life of the SEI Wealth Platform | 15 years |  |  |  |  |
Amortization expense related to the SEI Wealth Platform |  |  | 37,324 | 33,387 |  |
SEI Wealth Platform [Member] | Weighted Average [Member] |  |  |  |  |  |
Estimated useful life of the SEI Wealth Platform | 4 years 8 months 12 days |  |  |  |  |
Application for Investment Managers segment [Member] |  |  |  |  |  |
Capitalized software development costs |  |  | 7,969 | $ 5,809 |  |
Net book value of capitalized software | $ 23,477 |  | 23,477 |  |  |
SEI Investments (Europe) Limited [Member] |  |  |  |  |  |
Restricted cash | 3,000 |  | 3,000 |  | 3,000 |
SEI Investments Distribution Co. (SIDCO) [Member] |  |  |  |  |  |
Restricted cash | 503 |  | 503 |  | 500 |
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member] |  |  |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 298,869 |  | $ 298,869 |  | $ 374,760 |

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