Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information (Unaudited) [Abstract]
Revenues	$ 99,950	$ 87,721	$ 83,944	$ 81,385	$ 90,317	$ 92,160	$ 98,693	$ 99,806	$ 353,000	$ 380,976
Operating income	52,031	48,076	46,747	44,942	33,102	37,276	38,981	38,590	191,796	147,949	$ 169,452
Income from continuing operations					17,925	22,451	23,775	23,148	117,121	87,299	106,283
Income/(loss) from discontinued operations, net of taxes					1,642	(7,483)	326	1,628	0	(3,887)	3,107
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 32,692	$ 30,861	$ 27,543	$ 26,025	$ 19,567	$ 14,968	$ 24,101	$ 24,776	$ 117,121	$ 83,412	$ 109,390
Net income attributable to GAMCO Investors, Inc.'s shareholders per share [Abstract]
Basic - Continuing operations (in dollars per share)					$ 0.68	$ 0.90	$ 0.95	$ 0.92	$ 4.01	$ 3.43	$ 4.20
Basic - Discontinued operations (in dollars per share)					0.06	(0.30)	0.01	0.07	0	(0.15)	0.12
Basic - Total (in dollars per share)	$ 1.12	$ 1.06	$ 0.94	$ 0.89	0.74	0.60	0.96	0.99	4.01	3.28	4.32
Diluted - Continuing operations (in dollars per share)					0.67	0.89	0.94	0.91	3.92	3.40	4.16
Diluted - Discontinued operations (in dollars per share)					0.06	(0.30)	0.01	0.06	0	(0.15)	0.12
Diluted - Total (in dollars per share)	$ 1.07	$ 1.03	$ 0.93	$ 0.88	$ 0.73	$ 0.59	$ 0.95	$ 0.97	$ 3.92	$ 3.24	$ 4.28
Stock compensation expense resulting from accelerated lapsing of restrictions on RSAs					$ 3,500
Stock compensation expense resulting from accelerated lapsing of restrictions on RSAs per fully diluted share (in dollars per share)					$ 0.07