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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, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Expense related to soft dollar arrangement | $ 3,730 | $ 4,073 |
Cash and cash equivalents invested in SEI-sponsored money market funds | 748,299 |  | $ 744,247
Restricted cash | 3,507 |  | 3,505
Capitalized software development costs | 12,887 | $ 16,861 |
Net book value of capitalized software | $ 312,272 |  | 310,405
Anti-dilutive employee stock options | 6,054 | 11,279 |
Anti-dilutive employee stock options, per share | $ 52.73 | $ 37.70 |
SEI Wealth Platform [Member] |  |  |
Capitalized software development costs | $ 12,042 | $ 15,178 |
Net book value of capitalized software | 287,530 |  |
Capitalized software in progress | 33,237 |  |
Amortization expense of capitalized software | $ 9,719 | 11,972 |
SEI Wealth Platform [Member] | Weighted Average [Member] |  |  |
Estimated useful life of capitalized software | 8 years 4 months 24 days |  |
Application for Investment Managers segment [Member] |  |  |
Capitalized software development costs | $ 845 | $ 1,683 |
Net book value of capitalized software | $ 24,742 |  |
Estimated useful life of capitalized software | 5 years |  |
Amortization expense of capitalized software | $ 1,301 |  |
SEI Investments (Europe) Limited [Member] |  |  |
Restricted cash | 3,000 |  | 3,000
SEI Investments Distribution Co. (SIDCO) [Member] |  |  |
Restricted cash | 507 |  | 505
SEI-Sponsored Open-Ended Money Market Mutual Funds [Member] |  |  |
Cash and cash equivalents invested in SEI-sponsored money market funds | $ 341,413 |  | $ 401,292

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