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Summary of Significant Accounting Policies (Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Dec. 31, 2013
Dec. 31, 2012
Cash and cash equivalents invested in SEI-sponsored money market funds $ 532,167 $ 487,059 $ 532,167 $ 487,059 $ 578,273 $ 452,247
Restricted cash 5,500   5,500   5,500  
Net book value of the SEI Wealth Platform 312,193   312,193   312,615  
Anti-dilutive employee stock options 5,397,000 5,548,000 5,397,000 6,745,000    
Antidilutive securities excluded from computation of net income, per outstanding unit, amount $ 32.40 $ 29.22 $ 32.40 $ 28.01    
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 328,014   328,014   387,201  
SEI Wealth Platform [Member]
            
Capitalized software development costs related to the SEI Wealth Platform     18,262 22,186    
One-time contractual payment for conversion option included in software development costs       8,812    
Net book value of the SEI Wealth Platform 312,193   312,193      
Capitalized software development costs in-progress associated with future releases 8,854   8,854      
Estimated useful life of the SEI Wealth Platform 15 years          
Amortization expense related to the SEI Wealth Platform     $ 18,589 $ 16,321    
Weighted Average [Member] | SEI Wealth Platform [Member]
            
Estimated useful life of the SEI Wealth Platform 8 years