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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, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2019
Summary of Significant Accounting Policies [Line Items]                        
Cash and cash equivalents $ 841,446,000       $ 754,525,000       $ 841,446,000 $ 754,525,000    
Restricted cash 3,101,000       3,514,000       3,101,000 3,514,000    
Impairment charges related to marketable securities                 0 0 $ 0  
Capitalized software development costs                 34,074,000 44,221,000 61,043,000  
Net book value of capitalized software 296,068,000       309,500,000       296,068,000 309,500,000    
Increase in net income from change in estimate $ 128,737,000 $ 132,168,000 $ 126,540,000 $ 113,981,000 $ 116,034,000 $ 128,319,000 $ 121,677,000 $ 139,838,000 $ 501,426,000 $ 505,868,000 $ 404,389,000  
Increase in diluted earnings per share from change in estimate $ 0.84 $ 0.86 $ 0.82 $ 0.73 $ 0.73 $ 0.80 $ 0.75 $ 0.86 $ 3.24 $ 3.14 $ 2.49  
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)                 6,574 6,224 5,196  
Anti-dilutive employee stock options, average exercise price per share                 $ 55.95 $ 53.60 $ 45.49  
Operating lease, right-of-use asset $ 42,789,000               $ 42,789,000      
Operating lease liability 47,606,000               47,606,000      
SEI Wealth Platform                        
Summary of Significant Accounting Policies [Line Items]                        
Capitalized software development costs                 33,084,000 $ 43,376,000 $ 51,353,000  
Net book value of capitalized software 279,453,000               279,453,000      
Capitalized software development costs in progress 55,332,000               55,332,000      
Amortization expense related to the SEI Wealth Platform                 42,297,000 39,917,000 46,505,000  
Application for the Investment Managers segment                        
Summary of Significant Accounting Policies [Line Items]                        
Capitalized software development costs                   845,000 9,690,000  
Net book value of capitalized software 15,625,000               $ 15,625,000      
Estimated useful life of the SEI Wealth Platform                 3 years      
Amortization expense related to the SEI Wealth Platform                 $ 5,209,000 5,000    
Remaining Useful Life of SEI Wealth Platform | SEI Wealth Platform                        
Summary of Significant Accounting Policies [Line Items]                        
Decrease in amortization expense from change in estimate                     4,347,000  
Increase in net income from change in estimate                     $ 3,156,000  
Increase in diluted earnings per share from change in estimate                     $ 0.02  
Accounting Standards Update 2016-02                        
Summary of Significant Accounting Policies [Line Items]                        
Operating lease, right-of-use asset                       $ 44,169,000
Operating lease liability                       $ 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      
Minimum | Furniture and fixtures                        
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      
Maximum | Furniture and fixtures                        
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                 8 years 4 months 24 days      
SEI Sponsored Open Ended Money Market Mutual Funds                        
Summary of Significant Accounting Policies [Line Items]                        
Cash and cash equivalents 414,581,000       $ 315,840,000       $ 414,581,000 315,840,000    
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       $ 514,000       101,000 514,000    
SEI Sponsored Open Ended Money Market Mutual Funds                        
Summary of Significant Accounting Policies [Line Items]                        
Fees waived according to expense limitation agreements                 $ 27,544,000 $ 26,229,000 $ 27,434,000