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Summary of Significant Accounting Policies (Narrative) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Dec. 31, 2012
Cash and cash equivalents invested in SEI-sponsored money market funds $ 577,914 $ 527,214 $ 577,914 $ 527,214 $ 578,273 $ 452,247
Restricted cash 5,500   5,500   5,500  
Net book value of the SEI Wealth Platform 310,368   310,368   312,615  
Anti-dilutive employee stock options 4,411 3,042 4,416 5,522    
Antidilutive securities excluded from computation of net income, per outstanding unit, amount $ 32.38 $ 31.39 $ 32.37 $ 29.22    
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 374,292   374,292   387,201  
SEI Wealth Platform [Member]
            
Capitalized software development costs related to the SEI Wealth Platform     26,193 31,044    
One-time contractual payment for conversion option included in software development costs       8,812    
Net book value of the SEI Wealth Platform 310,368   310,368      
Estimated useful life of the SEI Wealth Platform 15 years          
Amortization expense related to the SEI Wealth Platform     $ 28,345 $ 25,046    
Weighted Average [Member] | SEI Wealth Platform [Member]
            
Estimated useful life of the SEI Wealth Platform 7 years 9 months