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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
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