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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Summary of Significant Accounting Policies [Line Items]      
Cash and cash equivalents $ 834,697,000 $ 853,008,000  
Restricted cash 301,000 351,000  
Impairment charge on reclassified assets 0 0 $ 0
Capitalized software development costs 33,958,000 35,293,000 26,037,000
Net book value of capitalized software 239,783,000 237,302,000  
Impairment charges related to capitalized software development costs 5,250,000 0 0
Goodwill and intangible assets impairment $ 0 $ 0 $ 0
Anti-dilutive employee stock options (in shares) 11,388 12,439 11,755
Anti-dilutive employee stock options, average exercise price per share $ 61.32 $ 60.35 $ 58.43
Investment Managers      
Summary of Significant Accounting Policies [Line Items]      
Capitalized software development costs $ 15,775,000 $ 9,558,000  
Capitalized software development costs in progress 20,083,000    
SEI Wealth Platform      
Summary of Significant Accounting Policies [Line Items]      
Capitalized software development costs 18,183,000 25,735,000 $ 25,938,000
Net book value of capitalized software 218,521,000    
Capitalized software development costs in progress 5,916,000    
Amortization expense related to the SEI Wealth Platform $ 25,637,000 35,638,000 47,769,000
Weighted Average | SEI Wealth Platform      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life of the SEI Wealth Platform 8 years 10 months 24 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 $ 397,838,000 305,434,000  
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 51,000 101,000  
SEI Sponsored Open Ended Money Market Mutual Funds      
Summary of Significant Accounting Policies [Line Items]      
Fees waived according to expense limitation agreements $ 22,092,000 $ 35,426,000 $ 45,129,000