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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Summary of Significant Accounting Policies [Line Items]      
Cash and cash equivalents invested in SEI-sponsored money market funds $ 846,628   $ 834,697
Restricted cash 301   301
Capitalized software development costs 6,277 $ 9,041  
Net book value of capitalized software $ 239,156   239,783
Anti-dilutive employee stock options (in shares) 10,912 11,273  
Anti-dilutive employee stock options (in USD per share) $ 61.05 $ 61.28  
Investment Managers      
Summary of Significant Accounting Policies [Line Items]      
Capitalized software development costs $ 2,482 $ 4,307  
Capitalized software in progress 22,650   20,083
SEI Wealth Platform      
Summary of Significant Accounting Policies [Line Items]      
Capitalized software development costs 3,795 4,734  
Net book value of capitalized software 215,474    
Capitalized software in progress 3,795   5,916
Amortization expense related to the SEI Wealth Platform $ 6,757 6,158  
SEI Wealth Platform | Weighted Average      
Summary of Significant Accounting Policies [Line Items]      
Estimated useful life of the SEI Wealth Platform 8 years 9 months 18 days    
SEI-Sponsored Open-Ended Money Market Mutual Funds      
Summary of Significant Accounting Policies [Line Items]      
Cash and cash equivalents invested in SEI-sponsored money market funds $ 436,130   397,838
SEI Investments (Europe) Limited      
Summary of Significant Accounting Policies [Line Items]      
Restricted cash 250   250
SEI Investments Distribution Co. (SIDCO)      
Summary of Significant Accounting Policies [Line Items]      
Restricted cash 51   $ 51
SEI-Sponsored Open-Ended Money Market Mutual Funds      
Summary of Significant Accounting Policies [Line Items]      
Fees waived $ 4,299 $ 5,561