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Summary of Significant Accounting Policies (Narrative) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2013
Software Development [Member]
Dec. 31, 2012
Software Development [Member]
Dec. 31, 2011
Software Development [Member]
Dec. 31, 2013
SEI Wealth Platform [Member]
Dec. 31, 2012
SEI Wealth Platform [Member]
Dec. 31, 2011
SEI Wealth Platform [Member]
Dec. 31, 2012
SEI Wealth Platform [Member]
Amortization [Member]
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Sei Investments Europe Limited [Member]
Dec. 31, 2012
Sei Investments Europe Limited [Member]
Dec. 31, 2013
Sei Investments Distribution Co Sidco [Member]
Dec. 31, 2012
Sei Investments Distribution Co Sidco [Member]
Dec. 31, 2013
Minimum [Member]
Software Development [Member]
Dec. 31, 2013
Maximum [Member]
Dec. 31, 2013
Maximum [Member]
Software Development [Member]
Dec. 31, 2013
Weighted Average [Member]
Software Development [Member]
Dec. 31, 2013
Weighted Average [Member]
SEI Wealth Platform [Member]
Dec. 31, 2013
Building and Building Improvements [Member]
Minimum [Member]
Dec. 31, 2013
Building and Building Improvements [Member]
Maximum [Member]
Dec. 31, 2013
Computer Equipment [Member]
Minimum [Member]
Dec. 31, 2013
Computer Equipment [Member]
Maximum [Member]
Dec. 31, 2013
Furniture and Fixtures [Member]
Minimum [Member]
Dec. 31, 2013
Furniture and Fixtures [Member]
Maximum [Member]
Mar. 28, 2013
SEI Asset Korea Co., Ltd. [Member]
Dec. 31, 2013
SEI Asset Korea Co., Ltd. [Member]
Apr. 30, 2013
LSV Asset Management [Member]
Dec. 31, 2011
LSV Asset Management [Member]
Dec. 31, 2013
Equity Method Investee [Member]
Minimum [Member]
Dec. 31, 2013
Equity Method Investee [Member]
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]                                                                    
Percentage of ownership before sale of subsidiary                                                         56.10%          
Closing date, sale of subsidiary                                                           Mar. 28, 2013        
Equity Method Investment, Ownership Percentage                                     50.00%                       39.30% 41.20% 20.00% 50.00%
Cash and cash equivalents $ 578,273 $ 452,247 $ 420,986 $ 496,292               $ 387,201 $ 247,314                                          
Restricted cash                           5,000 5,000 500 1,000                                  
Property, Plant and Equipment, Useful Life                                             25 years 39 years 3 years 5 years 3 years 5 years            
Net book value of the Platform 312,615 307,490           309,725                                                    
Capitalized software, accumulated amortization 180,062 149,747           149,924                                                    
Capitalized software development costs in-progress associated with future releases               2,796                                                    
Additions to capitalized software (39,500) (31,004) (40,954)         (8,812)                                                    
Finite-Lived Intangible Asset, Useful Life               15 years                   3 years   15 years 8 years 6 months 8 years 6 months                        
Amortization expense related to capitalized software         34,375 32,647 26,153 34,045 32,177 25,610                                                
Capitalized computer software impairments                     $ 2,661                                              
Anti-dilutive employee stock options 7,736 13,202 15,914                                                              
Antidilutive Securities Excluded from Computation of Net Income, Per Outstanding Unit, Amount $ 30.54 $ 24.76 $ 23.07