Cover
| 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 |