Investment in Securities, Reclassifications Out of Accumulated Other Comprehensive Income into Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain from investments	$ (5,347)	$ 40
Other operating expenses/net gain from investments	5,453	5,119
Income before income taxes	35,201	40,120
Income tax provision	(7,940)	(15,300)
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 27,261	24,820
Amount Reclassified from AOCI [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes		0
Income tax provision		0
Net income attributable to GAMCO Investors, Inc.'s shareholders		0
Realized Gain on Sale of AFS Securities [Member] | Amount Reclassified from AOCI [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain from investments		0
Realized Gain on Donation of AFS Securities [Member] | Amount Reclassified from AOCI [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other operating expenses/net gain from investments		$ 0