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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2019
Summary of Significant Accounting Policies [Line Items]                        
Cash and cash equivalents $ 784,626,000       $ 841,446,000       $ 784,626,000 $ 841,446,000    
Restricted cash 3,101,000       3,101,000       3,101,000 3,101,000    
Impairment charges related to marketable securities                 0 0 $ 0  
Capitalized software development costs                 24,119,000 34,074,000 44,221,000  
Net book value of capitalized software 270,977,000       296,068,000       270,977,000 296,068,000    
Increase in net income from change in estimate $ 125,882,000 $ 111,096,000 $ 101,066,000 $ 109,242,000 $ 128,737,000 $ 132,168,000 $ 126,540,000 $ 113,981,000 $ 447,286,000 $ 501,426,000 $ 505,868,000  
Increase in diluted earnings per share from change in estimate $ 0.86 $ 0.75 $ 0.68 $ 0.72 $ 0.84 $ 0.86 $ 0.82 $ 0.73 $ 3.00 $ 3.24 $ 3.14  
Impairment charges related to capitalized software development costs                 $ 0      
Goodwill and intangible assets impairment                 $ 0      
Anti-dilutive employee stock options (in shares)                 9,220 6,574 6,224  
Anti-dilutive employee stock options, average exercise price per share                 $ 58.12 $ 55.95 $ 53.60  
Operating lease, right-of-use asset $ 38,397,000       $ 42,789,000       $ 38,397,000 $ 42,789,000    
SEI Wealth Platform                        
Summary of Significant Accounting Policies [Line Items]                        
Capitalized software development costs                 22,257,000 33,084,000 $ 43,376,000  
Net book value of capitalized software 257,709,000               257,709,000      
Capitalized software development costs in progress 13,409,000               13,409,000      
Amortization expense related to the SEI Wealth Platform                 43,853,000 42,297,000 39,917,000  
Application for the Investment Managers segment                        
Summary of Significant Accounting Policies [Line Items]                        
Capitalized software development costs                     845,000  
Net book value of capitalized software 10,416,000               $ 10,416,000      
Estimated useful life of the SEI Wealth Platform                 2 years      
Amortization expense related to the SEI Wealth Platform                 $ 5,209,000 5,209,000    
Accounting Standards Update 2016-02                        
Summary of Significant Accounting Policies [Line Items]                        
Operating lease, right-of-use asset                       $ 44,169,000
Minimum | Building and Building Improvements                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 25 years      
Minimum | Computer Equipment                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 3 years      
Maximum | Building and Building Improvements                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 39 years      
Maximum | Computer Equipment                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 5 years      
Weighted Average | SEI Wealth Platform                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of the SEI Wealth Platform                 9 years 10 months 24 days      
SEI Sponsored Open Ended Money Market Mutual Funds                        
Summary of Significant Accounting Policies [Line Items]                        
Cash and cash equivalents 347,082,000       414,581,000       $ 347,082,000 414,581,000    
SEI Investments Europe Limited                        
Summary of Significant Accounting Policies [Line Items]                        
Restricted cash 3,000,000       3,000,000       3,000,000 3,000,000    
SEI Investments Distribution Co                        
Summary of Significant Accounting Policies [Line Items]                        
Restricted cash $ 101,000       $ 101,000       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                 $ 33,408,000 $ 27,544,000 $ 26,229,000