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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, 2016 | Mar. 31, 2015 | Dec. 31, 2015 | Dec. 31, 2014
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 638,253 | $ 643,260 | $ 679,661 | $ 667,446
Restricted cash | 5,500 |  | 5,500 |
Capitalized software development costs | 9,477 | $ 7,989 |  |
Net book value of SEI Wealth Platform | $ 289,044 |  | 290,522 |
Anti-dilutive employee stock options | 10,506 | 10,051 |  |
Antidilutive securities excluded from computation of net income, per outstanding unit, amount | $ 34.05 | $ 30.02 |  |
SEI Investments Distribution Co. (SIDCO) [Member] |  |  |  |
Restricted cash | $ 500 |  | 500 |
SEI Investments (Europe) Limited [Member] |  |  |  |
Restricted cash | 5,000 |  | 5,000 |
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member] |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | 402,910 |  | $ 448,957 |
SEI Wealth Platform [Member] |  |  |  |
Capitalized software development costs | 7,645 | $ 7,330 |  |
Net book value of SEI Wealth Platform | $ 282,311 |  |  |
Estimated useful life of the SEI Wealth Platform | 15 years |  |  |
Amortization expense related to the SEI Wealth Platform | $ 10,955 | 10,301 |  |
Technological capabilities for Investment Managers segment [Member] |  |  |  |
Capitalized software development costs | $ 1,832 | $ 659 |  |
Weighted Average [Member] | SEI Wealth Platform [Member] |  |  |  |
Estimated useful life of the SEI Wealth Platform | 6 years 3 months |  |  |

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