Investment in Securities, Reclassifications Out of Accumulated Other Comprehensive Income into Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain from investments	$ 1,409	$ (14)	$ (3,938)	$ 26
Other operating expenses/net gain from investments	5,534	5,352	10,987	10,471
Income before income taxes	42,386	37,448	77,587	77,568
Income tax provision	(10,804)	(14,554)	(18,744)	(29,854)
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 31,582	22,894	$ 58,843	47,714
Amount Reclassified from AOCI [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes		34		34
Income tax provision		(13)		(13)
Net income attributable to GAMCO Investors, Inc.'s shareholders		21		21
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		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		$ 34		$ 34