Cover Page - shares	6 Months Ended
	Jun. 30, 2021	Jul. 22, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
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 $0.01 per share
Trading Symbol	SEIC
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		141,076,628
Entity Central Index Key	0000350894
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Amendment Flag	false
Current Fiscal Year End Date	--12-31