| Document and Entity Information - shares | Mar. 31, 2021 | Apr. 30, 2021 |
| Cover [Abstract] | ||
| Document Type | 10-Q/A | |
| Amendment Flag | false | |
| Document Quarterly Report | true | |
| Document Period End Date | Mar. 31, 2021 | |
| Current Fiscal Year End Date | --12-31 | |
| Document Transition Report | false | |
| 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 | |
| Entity Filer Category | Accelerated Filer | |
| Entity Current Reporting Status | Yes | |
| Entity Interactive Data Current | Yes | |
| Entity Emerging Growth Company | false | |
| Entity Shell Company | false | |
| Entity Small Business | true | |
| Entity Common Stock, Shares Outstanding | 8,290,951 | |
| Document Fiscal Year Focus | 2021 | |
| Document Fiscal Period Focus | Q1 | |
| 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 |
