XML 11 R1.htm IDEA: XBRL DOCUMENT v3.21.2
Cover
Nov. 12, 2021
Nov. 10, 2021
Cover [Abstract]    
Document Type 8-K  
Document Period End Date Nov. 12, 2021  
Entity Registrant Name SEI INVESTMENTS COMPANY  
Entity Central Index Key   0000350894
Amendment Flag   false
Entity Incorporation, State or Country Code PA  
Entity File Number 0-10200  
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  
Written Communications false  
Soliciting Material false  
Pre-commencement Tender Offer false  
Pre-commencement Issuer Tender Offer false  
Title of 12(b) Security Common Stock, par value $0.01 per share  
Trading Symbol SEIC  
Security Exchange Name NASDAQ  
Entity Emerging Growth Company false