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Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 3 Months Ended |  |  |  |  |  |  |  |  | 3 Months Ended |
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Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2012 | Dec. 31, 2011 | Mar. 31, 2013 SEI-Sponsored Open-Ended Money Market Mutual Funds [Member] | Dec. 31, 2012 SEI-Sponsored Open-Ended Money Market Mutual Funds [Member] | Mar. 31, 2013 SEI Investments (Europe) Limited [Member] | Dec. 31, 2012 SEI Investments (Europe) Limited [Member] | Mar. 31, 2013 SEI Investments Distribution Co. (SIDCO) [Member] | Dec. 31, 2012 SEI Investments Distribution Co. (SIDCO) [Member] | Mar. 31, 2013 SEI Wealth Platform [Member] | Mar. 31, 2012 SEI Wealth Platform [Member] | Mar. 28, 2013 SEI Asset Korea Co., Ltd. [Member] | Mar. 31, 2013 SEI Asset Korea Co., Ltd. [Member] | Dec. 31, 2012 SEI Asset Korea Co., Ltd. [Member]
Percentage of ownership before sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  | 56.10% |  |
Cash and cash equivalents | $ 452,099 | $ 388,127 | $ 452,247 | $ 420,986 | $ 308,700 | $ 247,314 |  |  |  |  |  |  |  |  | $ 48,306
Restricted cash |  |  |  |  |  |  | 5,000 | 5,000 | 500 | 1,000 |  |  |  |  |
Payments to acquire software | 6,048 | 9,277 |  |  |  |  |  |  |  |  |  |  |  |  |
Capitalized Software Weighted Average Remaining Life | 9 years 3 months |  |  |  |  |  |  |  |  |  | 9 years 3 months |  |  |  |
Amortization expense related to capitalized software | 8,186 | 7,181 |  |  |  |  |  |  |  |  | 8,068 | 7,064 |  |  |
Net book value of SWP |  |  |  |  |  |  |  |  |  |  | 303,026 |  |  |  |
Capitalized software, accumulated amortization | 157,933 |  | 149,747 |  |  |  |  |  |  |  | 128,007 |  |  |  |
Capitalized software development costs in-progress associated with future releases |  |  |  |  |  |  |  |  |  |  | $ 2,020 |  |  |  |
Software Development Cost Useful Life |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |  |
Anti-dilutive employee stock options | 6,863,000 | 14,329,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Antidilutive Securities Excluded from Computation of Net Income, Per Outstanding Unit, Amount | $ 28.05 | $ 23.43 |  |  |  |  |  |  |  |  |  |  |  |  |
Closing date, sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  | Mar. 28, 2013 |

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