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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, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Dec. 31, 2014 | Dec. 31, 2013
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 647,589 | $ 577,914 | $ 647,589 | $ 577,914 | $ 667,446 | $ 578,273
Restricted cash | 6,319 |  | 6,319 |  | 5,801 |
Capitalized software development costs |  |  | 22,508 | $ 26,193 |  |
Net book value of SEI Wealth Platform | $ 294,380 |  | $ 294,380 |  | 309,040 |
Anti-dilutive employee stock options | 9,938 | 4,411 | 9,999 | 4,416 |  |
Antidilutive securities excluded from computation of net income, per outstanding unit, amount | $ 30.02 | $ 32.38 | $ 30.02 | $ 32.37 |  |
SEI Investments Distribution Co. (SIDCO) [Member] |  |  |  |  |  |
Restricted cash | $ 500 |  | $ 500 |  | 500 |
SEI Investments (Europe) Limited [Member] |  |  |  |  |  |
Restricted cash | 5,000 |  | 5,000 |  | 5,000 |
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member] |  |  |  |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | 401,634 |  | 401,634 |  | $ 435,268 |
SEI Wealth Platform [Member] |  |  |  |  |  |
Capitalized software development costs |  |  | 19,344 | $ 26,193 |  |
Net book value of SEI Wealth Platform | $ 291,216 |  | 291,216 |  |  |
Estimated useful life of the SEI Wealth Platform | 15 years |  |  |  |  |
Amortization expense related to the SEI Wealth Platform |  |  | $ 31,635 | $ 28,345 |  |
Income statement classification for capitalized software write off related to the SEI Wealth Platform | Facilities, supplies and other costs |  |  |  |  |
Weighted Average [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |
Estimated useful life of the SEI Wealth Platform | 6 years 9 months |  |  |  |  |
Capitalized software [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |
Loss on write off of capitalized software related to the SEI Wealth Platform | $ 5,533 |  |  |  |  |
Private Banks [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |
Segment classification for capitalized software write off | Private Banks |  |  |  |  |
Investment Advisors [Member] | SEI Wealth Platform [Member] |  |  |  |  |  |
Segment classification for capitalized software write off | Investment Advisors |  |  |  |  |

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