Investment in Securities, Reclassifications Out of Accumulated Other Comprehensive Income into Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassifications out of accumulated other comprehensive income ("AOCI") into income [Abstract]
Net gain from investments	$ (4,328)	$ 2,841	$ (8,090)	$ 2,867
Other operating expenses/net gain from investments	5,258	5,429	16,245	15,900
Income before income taxes	46,436	20,434	124,023	98,002
Income tax provision	(11,420)	(3,834)	(30,164)	(33,688)
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 35,016	16,600	$ 93,859	64,314
Amount Reclassified from AOCI [Member]
Reclassifications out of accumulated other comprehensive income ("AOCI") into income [Abstract]
Income before income taxes		2,841		2,875
Income tax provision		(1,051)		(1,064)
Net income attributable to GAMCO Investors, Inc.'s shareholders		1,790		1,811
Realized Gain on Sale of AFS Securities [Member] | Amount Reclassified from AOCI [Member]
Reclassifications out of accumulated other comprehensive income ("AOCI") into income [Abstract]
Net gain from investments		20		20
Realized Gain on Donation of AFS Securities [Member] | Amount Reclassified from AOCI [Member]
Reclassifications out of accumulated other comprehensive income ("AOCI") into income [Abstract]
Other operating expenses/net gain from investments		$ 2,821		$ 2,855