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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Summary of Significant Accounting Policies [Line Items]      
Cash and cash equivalents $ 831,407,000 $ 784,626,000  
Restricted cash 351,000 3,101,000  
Impairment charges related to marketable securities 0 0 $ 0
Capitalized software development costs 26,037,000 24,119,000 34,074,000
Net book value of capitalized software 243,446,000 270,977,000  
Impairment charges related to capitalized software development costs 0 0 0
Goodwill and intangible assets impairment $ 0 $ 0 $ 0
Anti-dilutive employee stock options (in shares) 11,755 9,220 6,574
Anti-dilutive employee stock options, average exercise price per share $ 58.43 $ 58.12 $ 55.95
SEI Wealth Platform      
Summary of Significant Accounting Policies [Line Items]      
Capitalized software development costs $ 25,938,000 $ 22,257,000 $ 33,084,000
Net book value of capitalized software 235,878,000    
Capitalized software development costs in progress 29,253,000    
Amortization expense related to the SEI Wealth Platform $ 47,769,000 43,853,000 42,297,000
Weighted Average | SEI Wealth Platform      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life of the SEI Wealth Platform 10 years 1 month 6 days    
Building and Building Improvements | Minimum      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life of property and equipment 25 years    
Building and Building Improvements | Maximum      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life of property and equipment 39 years    
Computer Equipment | Minimum      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life of property and equipment 3 years    
Computer Equipment | Maximum      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life of property and equipment 5 years    
SEI Sponsored Open Ended Money Market Mutual Funds      
Summary of Significant Accounting Policies [Line Items]      
Cash and cash equivalents $ 290,256,000 347,082,000  
SEI Investments Europe Limited      
Summary of Significant Accounting Policies [Line Items]      
Restricted cash 250,000 250,000  
SEI Investments Distribution Co      
Summary of Significant Accounting Policies [Line Items]      
Restricted cash 101,000 101,000  
SEI Sponsored Open Ended Money Market Mutual Funds      
Summary of Significant Accounting Policies [Line Items]      
Fees waived according to expense limitation agreements $ 45,129,000 $ 33,408,000 $ 27,544,000