Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic [Abstract]
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 35,016	$ 16,600	$ 93,859	$ 64,314
Weighted average share outstanding (in shares)	28,677	28,926	28,789	28,930
Basic net income per share attributable to GAMCO Investors, Inc.'s shareholders (in dollars per share)	$ 1.22	$ 0.57	$ 3.26	$ 2.22
Diluted [Abstract]
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 35,016	$ 16,600	$ 93,859	$ 64,314
Income from continuing operations			93,859	64,314
Add interest on convertible notes, net of management fee and taxes	0	696	0	2,192
Total income attributable to GAMCO Investors, Inc.'s shareholders	$ 35,016	$ 17,296	$ 93,859	$ 66,506
Weighted average share outstanding (in shares)	28,677	28,926	28,789	28,930
Restricted stock awards (in shares)	62	247	35	214
Assumed conversion of convertible notes (in shares)	0	2,000	0	2,000
Total (in shares)	28,739	31,173	28,824	31,144
Diluted net income per share attributable to GAMCO Investors, Inc.'s shareholders (in dollars per share)	$ 1.22	$ 0.55	$ 3.26	$ 2.14