Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Basic [Abstract]
Income from continuing operations					$ 17,925	$ 22,451	$ 23,775	$ 23,148	$ 117,121	$ 87,299	$ 106,283
Gain/(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
Weighted average shares outstanding (in shares)									29,182	25,425	25,335
Basic net income per share attributable to GAMCO Investors, Inc.'s shareholders [Abstract]
Continuing operations (in dollars per share)					$ 0.68	$ 0.90	$ 0.95	$ 0.92	$ 4.01	$ 3.43	$ 4.20
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 [Abstract]
Income from continuing operations					$ 17,925	$ 22,451	$ 23,775	$ 23,148	$ 117,121	$ 87,299	$ 106,283
Add interest on convertible notes, net of management fee and taxes									1,133	0	0
Total income from continuing operations									118,254	87,299	106,283
Gain/(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									$ 118,254	$ 83,412	$ 109,390
Weighted average shares outstanding (in shares)									29,182	25,425	25,335
Dilutive stock options and restricted stock awards (in shares)									234	286	223
Assumed conversion of convertible notes (in shares)									754	0	0
Total (in shares)									30,170	25,711	25,558
Diluted net income per share attributable to GAMCO Investors, Inc.'s shareholder [Abstract]
Continuing operations (in dollars per share)					$ 0.67	$ 0.89	$ 0.94	$ 0.91	$ 3.92	$ 3.40	$ 4.16
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