Investments in Partnerships, Offshore Funds and Variable Interest Entities, Part II (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets [Abstract]
Cash and cash equivalents at end of period	$ 210,451				$ 190,608				$ 210,451	$ 190,608	$ 276,340	$ 169,601
Investments in securities	231,228				218,843				231,228	218,843
Investments in sponsored investment companies	44,042				61,872				44,042	61,872
Investments in partnerships	95,992				97,549				95,992	97,549
Receivable from brokers	49,461				50,655				49,461	50,655
Total assets	709,485				690,733				709,485	690,733
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	6,178				3,136				6,178	3,136
Redeemable noncontrolling interests	6,751				17,362				6,751	17,362
Total equity	460,182				370,934				460,182	370,934	407,411	389,608
Total liabilities and equity	709,485				690,733				709,485	690,733
Condensed Consolidated Statements of Income [Abstract]
Total revenues									397,562	344,281	327,128
Total expenses									255,150	233,151	213,834
Operating income	47,173	32,728	31,784	30,727	24,739	29,012	30,367	27,012	142,412	111,130	113,294
Total other income, net									41,139	6,186	(2,852)
Income before income taxes									183,551	117,316	110,442
Income tax provision									66,186	41,721	40,767
Net income									117,365	75,595	69,675
Net income/(loss) attributable to noncontrolling interests									512	56	(7)
Net income attributable to GAMCO Investors, Inc.'s shareholders									116,853	75,539	69,682
VIE [Member]
Assets [Abstract]
Investments in partnerships	15,540				18,507				15,540	18,507
Liabilities and Equity [Abstract]
Accrued expense and other liabilities	(2,022)				(3,010)				(2,022)	(3,010)
Redeemable noncontrolling interests	(1,120)				(411)				(1,120)	(411)
GAMCO's net interests in consolidated VIEs	12,398				15,086				12,398	15,086
As Reported [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	210,451				190,608				210,451	190,608
Investments in securities	275,270				280,715				275,270	280,715
Investments in partnerships	95,992				97,549				95,992	97,549
Receivable from brokers	49,461				50,655				49,461	50,655
Investment advisory fees receivable	51,506				42,429				51,506	42,429
Other asset	26,805				28,777				26,805	28,777
Total assets	709,485				690,733				709,485	690,733
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	6,178				3,136				6,178	3,136
Accrued expense and other liabilities	124,463				82,935				124,463	82,935
Total Debt	111,911				216,366				111,911	216,366
Redeemable noncontrolling interests	6,751				17,362				6,751	17,362
Total equity	460,182				370,934				460,182	370,934
Total liabilities and equity	709,485				690,733				709,485	690,733
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	209,667				189,743				209,667	189,743
Investments in securities	276,244				275,491				276,244	275,491
Investments in partnerships	98,494				100,164				98,494	100,164
Receivable from brokers	35,151				25,972				35,151	25,972
Investment advisory fees receivable	52,509				42,425				52,509	42,425
Other asset	27,433				32,673				27,433	32,673
Total assets	699,498				666,468				699,498	666,468
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	6,049				3,033				6,049	3,033
Accrued expense and other liabilities	121,356				76,135				121,356	76,135
Total Debt	111,911				216,366				111,911	216,366
Redeemable noncontrolling interests	0				0				0	0
Total equity	460,182				370,934				460,182	370,934
Total liabilities and equity	699,498				666,468				699,498	666,468
Condensed Consolidated Statements of Income [Abstract]
Total revenues									399,565	346,195	328,151
Total expenses									254,153	232,313	212,844
Operating income									145,412	113,882	115,307
Total other income, net									37,676	3,264	(4,872)
Income before income taxes									183,088	117,146	110,435
Income tax provision									66,186	41,721	40,767
Net income									116,902	75,425	69,668
Net income/(loss) attributable to noncontrolling interests									49	(114)	(14)
Net income attributable to GAMCO Investors, Inc.'s shareholders									116,853	75,539	69,682
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	450				0				450	0
Investments in securities	0				0				0	0
Investments in partnerships	6,517				5,388				6,517	5,388
Receivable from brokers	0				0				0	0
Investment advisory fees receivable	(24)				9				(24)	9
Other asset	(2,339)				(2,986)				(2,339)	(2,986)
Total assets	4,604				2,411				4,604	2,411
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0				0				0	0
Accrued expense and other liabilities	165				384				165	384
Total Debt	0				0				0	0
Redeemable noncontrolling interests	4,439				2,027				4,439	2,027
Total equity	0				0				0	0
Total liabilities and equity	4,604				2,411				4,604	2,411
Condensed Consolidated Statements of Income [Abstract]
Total revenues									(53)	2	(24)
Total expenses									175	132	121
Operating income									(228)	(130)	(145)
Total other income, net									475	216	71
Income before income taxes									247	86	(74)
Income tax provision									0	0	0
Net income									247	86	(74)
Net income/(loss) attributable to noncontrolling interests									247	86	(74)
Net income attributable to GAMCO Investors, Inc.'s shareholders									0	0	0
Partnerships [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	334				865				334	865
Investments in securities	7,473				6,964				7,473	6,964
Investments in partnerships	(9,019)				(8,003)				(9,019)	(8,003)
Receivable from brokers	0				1,480				0	1,480
Investment advisory fees receivable	(14)				(5)				(14)	(5)
Other asset	1,592				(1,000)				1,592	(1,000)
Total assets	366				301				366	301
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0				0				0	0
Accrued expense and other liabilities	29				21				29	21
Total Debt	0				0				0	0
Redeemable noncontrolling interests	337				280				337	280
Total equity	0				0				0	0
Total liabilities and equity	366				301				366	301
Condensed Consolidated Statements of Income [Abstract]
Total revenues									(15)	(6)	(3)
Total expenses									47	39	40
Operating income									(62)	(45)	(43)
Total other income, net									111	67	34
Income before income taxes									49	22	(9)
Income tax provision									0	0	0
Net income									49	22	(9)
Net income/(loss) attributable to noncontrolling interests									49	22	(9)
Net income attributable to GAMCO Investors, Inc.'s shareholders									0	0	0
Offshore Funds [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	0				0				0	0
Investments in securities	(8,447)				(1,740)				(8,447)	(1,740)
Investments in partnerships	0				0				0	0
Receivable from brokers	14,310				23,203				14,310	23,203
Investment advisory fees receivable	(965)				0				(965)	0
Other asset	119				90				119	90
Total assets	5,017				21,553				5,017	21,553
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	129				103				129	103
Accrued expense and other liabilities	2,913				6,395				2,913	6,395
Total Debt	0				0				0	0
Redeemable noncontrolling interests	1,975				15,055				1,975	15,055
Total equity	0				0				0	0
Total liabilities and equity	5,017				21,553				5,017	21,553
Condensed Consolidated Statements of Income [Abstract]
Total revenues									(1,935)	(1,910)	(996)
Total expenses									775	667	829
Operating income									(2,710)	(2,577)	(1,825)
Total other income, net									2,877	2,639	1,915
Income before income taxes									167	62	90
Income tax provision									0	0	0
Net income									167	62	90
Net income/(loss) attributable to noncontrolling interests									167	62	90
Net income attributable to GAMCO Investors, Inc.'s shareholders									$ 0	$ 0	$ 0