| Cover | Mar. 04, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Mar. 4, 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 |
