Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic [Abstract]
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 27,261	$ 24,820
Weighted average share outstanding (in shares)	28,916	28,970
Basic net income per share attributable to GAMCO Investors, Inc.'s shareholders (in dollars per share)	$ 0.94	$ 0.86
Diluted [Abstract]
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 27,261	$ 24,820
Add interest on convertible notes, net of management fee and taxes	0	748
Total income attributable to GAMCO Investors, Inc.'s shareholders	$ 27,261	$ 25,568
Weighted average share outstanding (in shares)	28,916	28,970
Dilutive stock options and restricted stock awards	0	190
Assumed conversion of convertible notes (in shares)	0	2,000
Total (in shares)	28,916	31,160
Diluted net income per share attributable to GAMCO Investors, Inc.'s shareholders (in dollars per share)	$ 0.94	$ 0.82