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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, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2018
Dec. 31, 2015
Summary of Significant Accounting Policies [Line Items]                          
Expense related to soft dollar arrangement                 $ 16,680,000 $ 14,623,000 $ 18,409,000    
Cash and cash equivalents $ 754,525,000       $ 744,247,000       754,525,000 744,247,000      
Restricted cash 3,514,000       3,505,000       3,514,000 3,505,000      
Impairment charges related to marketable securities                 0 0 0    
Net book value of capitalized software $ 309,500,000       $ 310,405,000       309,500,000 310,405,000      
Increase in net income from change in estimate                 $ 505,868,000 $ 404,389,000 $ 333,817,000    
Increase in diluted earnings per share from change in estimate $ 0.73 $ 0.80 $ 0.75 $ 0.86 $ 0.75 $ 0.63 $ 0.57 $ 0.55 $ 3.14 $ 2.49 $ 2.03    
Capitalized software development costs                 $ 44,221,000 $ 61,043,000 $ 50,392,000    
Impairment charges related to capitalized software development costs                 0 0 $ 0    
Goodwill and intangible assets impairment                 $ 0 $ 0      
Anti-dilutive employee stock options                 6,224 5,196 10,632    
Anti-dilutive employee stock options, average exercise price per share                 $ 53.60 $ 45.49 $ 35.02    
SEI Wealth Platform                          
Summary of Significant Accounting Policies [Line Items]                          
Net book value of capitalized software $ 288,666,000               $ 288,666,000        
Capitalized software development costs in progress 42,238,000               42,238,000        
Capitalized software development costs                 43,376,000 $ 51,353,000 $ 39,785,000    
Amortization expense related to the SEI Wealth Platform                 39,917,000 46,505,000 45,047,000    
Application for the Investment Managers segment                          
Summary of Significant Accounting Policies [Line Items]                          
Net book value of capitalized software 20,834,000               20,834,000        
Capitalized software development costs                 $ 845,000 9,690,000 10,607,000    
Estimated useful life of the SEI Wealth Platform                 5 years        
Amortization expense related to the SEI Wealth Platform                 $ 5,209,000        
SEI Sponsored Open Ended Money Market Mutual Funds                          
Summary of Significant Accounting Policies [Line Items]                          
Cash and cash equivalents 315,840,000       $ 401,292,000       315,840,000 401,292,000      
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 $ 514,000       505,000       $ 514,000 505,000      
Minimum                          
Summary of Significant Accounting Policies [Line Items]                          
Contract acquisition costs, amortization period 6 years               6 years        
Maximum                          
Summary of Significant Accounting Policies [Line Items]                          
Ownership interest in unconsolidated affiliate 50.00%               50.00%        
Contract acquisition costs, amortization period 15 years               15 years        
Weighted Average | SEI Wealth Platform                          
Summary of Significant Accounting Policies [Line Items]                          
Estimated useful life of the SEI Wealth Platform                 8 years 6 months        
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        
Furniture and fixtures | Minimum                          
Summary of Significant Accounting Policies [Line Items]                          
Estimated useful life of property and equipment                 3 years        
Furniture and fixtures | Maximum                          
Summary of Significant Accounting Policies [Line Items]                          
Estimated useful life of property and equipment                 5 years        
Equity Method Investee | Minimum                          
Summary of Significant Accounting Policies [Line Items]                          
Ownership interest in unconsolidated affiliate 20.00%               20.00%        
Equity Method Investee | Maximum                          
Summary of Significant Accounting Policies [Line Items]                          
Ownership interest in unconsolidated affiliate 50.00%               50.00%        
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      
Retained Earnings                          
Summary of Significant Accounting Policies [Line Items]                          
Increase in net income from change in estimate                 $ 505,868,000 $ 404,389,000 333,817,000    
Accounting Standards Update 2014-09 | Retained Earnings                          
Summary of Significant Accounting Policies [Line Items]                          
Cumulative effect upon adoption of ASUs         $ 14,402,000         14,402,000 0 $ 14,402,000 $ 0
Cumulative effect adjustment, before tax                       $ 18,641,000  
SEI Sponsored Open Ended Money Market Mutual Funds                          
Summary of Significant Accounting Policies [Line Items]                          
Fees waived according to expense limitation agreements                 $ 26,229,000 $ 27,434,000 $ 41,227,000