Investment in Securities, Reclassifications Out of Accumulated Other Comprehensive Income into Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain from investments									$ 3,115	$ 1,594	$ 4,953
Other operating expenses									23,221	23,925	19,163
Income before income taxes									132,888	182,227	139,025
Income tax benefit									(55,079)	(65,106)	(51,726)
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 13,495	$ 16,600	$ 22,894	$ 24,820	$ 32,692	$ 30,861	$ 27,543	$ 26,025	77,809	117,121	$ 83,412
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes									3,125	1,255
Income tax benefit									(1,156)	(464)
Net income attributable to GAMCO Investors, Inc.'s shareholders									1,969	791
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									62	4
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									$ 3,063	$ 1,251