Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic [Abstract]
Income from continuing operations			$ 47,714	$ 53,568
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 22,894	$ 27,543	$ 47,714	$ 53,568
Weighted average shares outstanding (in shares)	28,896	29,234	28,933	29,241
Basic net income per share attributable to GAMCO Investors, Inc.'s shareholders (in dollars per share)	$ 0.79	$ 0.94	$ 1.65	$ 1.83
Diluted [Abstract]
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 22,894	$ 27,543	$ 47,714	$ 53,568
Income from continuing operations			47,714	53,568
Add interest on convertible notes, net of management fee and taxes	748	0	1,496	0
Total income attributable to GAMCO Investors, Inc.'s shareholders	$ 23,642	$ 27,543	$ 49,210	$ 53,568
Weighted average shares outstanding (in shares)	28,896	29,234	28,933	29,241
Restricted stock awards	204	288	197	269
Assumed conversion of convertible notes (in shares)	2,000	0	2,000	0
Total (in shares)	31,100	29,522	31,130	29,510
Diluted net income per share attributable to GAMCO Investors, Inc.'s shareholders (in dollars per share)	$ 0.76	$ 0.93	$ 1.58	$ 1.82