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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Summary of Significant Accounting Policies [Line Items]                        
Cash and cash equivalents $ 744,247       $ 695,701       $ 744,247 $ 695,701 $ 679,661 $ 667,446
Restricted cash 3,505       3,500       3,505 3,500    
Capitalized software development costs                 61,043 50,392 29,416  
Net book value of capitalized software $ 310,405       $ 295,867       310,405 295,867    
Increase in income from operations from change in estimate                 396,944 375,694 358,213  
Increase in net income from change in estimate                 $ 404,389 $ 333,817 $ 331,655  
Increase in diluted earnings per share from change in estimate $ 0.75 $ 0.63 $ 0.57 $ 0.55 $ 0.55 $ 0.53 $ 0.49 $ 0.47 $ 2.49 $ 2.03 $ 1.96  
Estimated amortization expense related to all capitalized software in 2018 $ 43,717               $ 43,717      
Estimated amortization expense related to all capitalized software in 2019 43,717               43,717      
Estimated amortization expense related to all capitalized software in 2020 43,717               43,717      
Estimated amortization expense related to all capitalizes software in 2021 43,439               43,439      
Estimated amortization expense related to all capitalized software in 2022 29,556               $ 29,556      
Anti-dilutive employee stock options                 5,196 10,632 10,730  
Anti-dilutive employee stock options, average exercise price per share                 $ 45.49 $ 35.02 $ 33.99  
SEI Wealth Platform [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Capitalized software development costs                 $ 51,353 $ 39,785 $ 24,515  
Net book value of capitalized software 285,207               285,207      
Capitalized software development costs in progress 38,093               $ 38,093      
Estimated useful life of the SEI Wealth Platform                 15 years      
Amortization expense related to the SEI Wealth Platform                 $ 46,505 45,047 $ 42,401  
Income statement classification for capitalized software write off related to the SEI Wealth Platform                     Facilities, supplies and other costs  
Application for the Investment Managers segment [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Capitalized software development costs                 9,690 10,607 $ 4,901  
Net book value of capitalized software 25,198               25,198      
SEI Investments Europe Limited [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Restricted cash 3,000       $ 3,000       3,000 3,000    
SEI Investments Distribution Co [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Restricted cash $ 505       500       $ 505 500    
Maximum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Ownership interest in unconsolidated affiliate 50.00%               50.00%      
Weighted Average [Member] | SEI Wealth Platform [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of the SEI Wealth Platform 8 years 2 months 12 days                      
Building and Building Improvements [Member] | Minimum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 25 years      
Building and Building Improvements [Member] | Maximum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 39 years      
Computer Equipment [Member] | Minimum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 3 years      
Computer Equipment [Member] | Maximum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 5 years      
Furniture and Fixtures [Member] | Minimum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 3 years      
Furniture and Fixtures [Member] | Maximum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Estimated useful life of property and equipment                 5 years      
Equity Method Investee [Member] | Minimum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Ownership interest in unconsolidated affiliate 20.00%               20.00%      
Equity Method Investee [Member] | Maximum [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Ownership interest in unconsolidated affiliate 50.00%               50.00%      
Capitalized software [Member] | SEI Wealth Platform [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Loss on write off of capitalized software related to the SEI Wealth Platform                     $ 5,533  
Private Banks [Member] | SEI Wealth Platform [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Segment classification for capitalized software write off related to the SEI Wealth Platform                     Private Banks  
Investment Advisors [Member] | SEI Wealth Platform [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Segment classification for capitalized software write off related to the SEI Wealth Platform                     Investment Advisors  
SEI Sponsored Open Ended Money Market Mutual Funds [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Cash and cash equivalents $ 401,292       $ 374,760       $ 401,292 $ 374,760    
Remaining Useful Life of SEI Wealth Platform [Member] | SEI Wealth Platform [Member]                        
Summary of Significant Accounting Policies [Line Items]                        
Decrease in amortization expense from change in estimate                 4,347      
Increase in income from operations from change in estimate                 4,347      
Increase in net income from change in estimate                 $ 3,156      
Increase in diluted earnings per share from change in estimate                 $ 0.02