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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, 2017 | Mar. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 661,372 | $ 638,253 | $ 695,701 | $ 679,661
Restricted cash | 3,500 |  | 3,500 |
Capitalized software development costs | 16,861 | $ 9,477 |  |
Net book value of SEI Wealth Platform | $ 300,756 |  | 295,867 |
Anti-dilutive employee stock options | 11,279 | 10,506 |  |
Antidilutive securities excluded from computation of net income, per outstanding unit, amount | $ 37.70 | $ 34.05 |  |
SEI Investments Distribution Co. (SIDCO) [Member] |  |  |  |
Restricted cash | $ 500 |  | 500 |
SEI Investments (Europe) Limited [Member] |  |  |  |
Restricted cash | 3,000 |  | 3,000 |
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member] |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | 313,762 |  | $ 374,760 |
SEI Wealth Platform [Member] |  |  |  |
Capitalized software development costs | 15,178 | $ 7,645 |  |
Net book value of SEI Wealth Platform | $ 283,565 |  |  |
Estimated useful life of the SEI Wealth Platform | 15 years |  |  |
Amortization expense related to the SEI Wealth Platform | $ 11,972 | 10,955 |  |
New application for Investment Managers segment [Member] |  |  |  |
Capitalized software development costs | $ 1,683 | $ 1,832 |  |
Weighted Average [Member] | SEI Wealth Platform [Member] |  |  |  |
Estimated useful life of the SEI Wealth Platform | 5 years 2 months 27 days |  |  |

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