Investment in Securities, Reclassifications Out of Accumulated Other Comprehensive Income into Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain/(loss) from investments	$ 223	$ 13
Other operating expenses/net gain from investments	4,372	4,663
Income before income taxes	42,127	37,226
Income tax provision	(16,102)	(14,078)
Net income attributable to GAMCO Investors, Inc.'s shareholders	26,025	24,776
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	2	3
Income tax provision	(1)	(1)
Net income attributable to GAMCO Investors, Inc.'s shareholders	1	2
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/(loss) from investments	2	3
Realized Gain on 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/net gain from investments	$ 0	$ 0