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Cover - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2020
Jan. 29, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 0-10200    
Entity Registrant Name SEI INVESTMENTS COMPANY    
Entity Incorporation, State or Country Code PA    
Entity Tax Identification Number 23-1707341    
Entity Address, Address Line One 1 Freedom Valley Drive    
Entity Address, City or Town Oaks    
Entity Address, State or Province PA    
Entity Address, Postal Zip Code 19456    
City Area Code 610    
Local Phone Number 676-1000    
Title of 12(b) Security Common Stock, par value $.01 per share    
Trading Symbol SEIC    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
ICFR Auditor Attestation Flag true    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 6.3
Entity Common Stock, Shares Outstanding   143,516,670  
Documents Incorporated by Reference
Portions of the following documents are incorporated by reference herein:

1.    The definitive proxy statement relating to the registrant’s 2021 Annual Meeting of Shareholders, to be filed within 120 days after the end of the fiscal year covered by this annual report, is incorporated by reference in Part III hereof.
    
Entity Central Index Key 0000350894    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Amendment Flag false