Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic:
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 14,969	$ 23,665	$ 63,846	$ 80,569
Weighted average shares outstanding (in shares)	24,947	25,296	25,047	25,385
Basic net income attributable to GAMCO Investors, Inc.'s shareholders per share (in dollars per share)	$ 0.60	$ 0.94	$ 2.55	$ 3.17
Diluted:
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 14,969	$ 23,665	$ 63,846	$ 80,569
Weighted average shares outstanding (in shares)	24,947	25,296	25,047	25,385
Dilutive stock options and restricted stock awards (in shares)	294	221	290	210
Total (in shares)	25,241	25,517	25,337	25,595
Diluted net income attributable to GAMCO Investors, Inc.'s shareholders per share (in dollars per share)	$ 0.59	$ 0.93	$ 2.52	$ 3.15