Investment in Securities, Reclassifications Out of Accumulated Other Comprehensive Income into Income (Details) - USD ($) $ 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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain from investments									$ 1,594	$ 4,953	$ 4,282
Other operating expenses									23,925	19,163	17,542
Income before income taxes									182,227	139,025	168,017
Income tax benefit									(65,106)	(51,726)	(61,734)
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
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes									1,255	4,491
Income tax benefit									(464)	(1,662)
Net income attributable to GAMCO Investors, Inc.'s shareholders									791	2,829
Realized Gain on Sale of AFS Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain from investments									4	6
Donation of AFS Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other operating expenses									$ 1,251	$ 4,485