| Document and Entity Information | Jul. 20, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jul. 20, 2021 |
| Current Fiscal Year End Date | --12-31 |
| Entity Registrant Name | GAMCO INVESTORS, INC. ET AL |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-14761 |
| Entity Tax Identification Number | 13-4007862 |
| Entity Address, Address Line One | 191 Mason Street |
| Entity Address, City or Town | Greenwich |
| Entity Address, State or Province | CT |
| Entity Address, Postal Zip Code | 06830 |
| City Area Code | 203 |
| Local Phone Number | 629-2726 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001060349 |
| Title of 12(b) Security | Class A Common Stock, $0.001 par value |
| Trading Symbol | GBL |
| Security Exchange Name | NYSE |
