Investments in Partnerships, Offshore Funds and Variable Interest Entities (Details) (USD $)	3 Months Ended		6 Months Ended				6 Months Ended						3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			6 Months Ended						3 Months Ended		6 Months Ended			6 Months Ended						3 Months Ended		6 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2012 VIE [Member]	Dec. 31, 2011 VIE [Member]	Jun. 30, 2011 VIE [Member]	Jun. 30, 2012 VOE [Member]	Dec. 31, 2011 VOE [Member]	Jun. 30, 2011 VOE [Member]	Jun. 30, 2012 Prior to Consolidations [Member]	Jun. 30, 2011 Prior to Consolidations [Member]	Jun. 30, 2012 Prior to Consolidations [Member]	Jun. 30, 2011 Prior to Consolidations [Member]	Dec. 31, 2011 Prior to Consolidations [Member]	Jun. 30, 2012 CFFs [Member]	Jun. 30, 2011 CFFs [Member]	Jun. 30, 2012 CFFs [Member]	Jun. 30, 2011 CFFs [Member]	Dec. 31, 2011 CFFs [Member]	Jun. 30, 2012 CFFs [Member] VIE [Member]	Dec. 31, 2011 CFFs [Member] VIE [Member]	Jun. 30, 2011 CFFs [Member] VIE [Member]	Jun. 30, 2012 CFFs [Member] VOE [Member]	Dec. 31, 2011 CFFs [Member] VOE [Member]	Jun. 30, 2011 CFFs [Member] VOE [Member]	Jun. 30, 2012 Partnerships [Member]	Jun. 30, 2011 Partnerships [Member]	Jun. 30, 2012 Partnerships [Member]	Jun. 30, 2011 Partnerships [Member]	Dec. 31, 2011 Partnerships [Member]	Jun. 30, 2012 Partnerships [Member] VIE [Member]	Dec. 31, 2011 Partnerships [Member] VIE [Member]	Jun. 30, 2011 Partnerships [Member] VIE [Member]	Jun. 30, 2012 Partnerships [Member] VOE [Member]	Dec. 31, 2011 Partnerships [Member] VOE [Member]	Jun. 30, 2011 Partnerships [Member] VOE [Member]	Jun. 30, 2012 Offshore Funds [Member]	Jun. 30, 2011 Offshore Funds [Member]	Jun. 30, 2012 Offshore Funds [Member]	Jun. 30, 2011 Offshore Funds [Member]	Dec. 31, 2011 Offshore Funds [Member]	Jun. 30, 2012 Offshore Funds [Member] VIE [Member]	Dec. 31, 2011 Offshore Funds [Member] VIE [Member]	Jun. 30, 2011 Offshore Funds [Member] VIE [Member]	Jun. 30, 2012 Offshore Funds [Member] VOE [Member]	Dec. 31, 2011 Offshore Funds [Member] VOE [Member]	Jun. 30, 2011 Offshore Funds [Member] VOE [Member]
Variable Interest Entity [Line Items]
Investments in affiliated entities	$ 88,000,000	$ 78,300,000	$ 88,000,000	$ 78,300,000	$ 86,900,000
Investments in unaffiliated entities	14,100,000	20,300,000	14,100,000	20,300,000	14,000,000
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period							1	2	2	4	4	4											1	1	1	2	2	2						0	0	0	1	1	2						0	1	1	1	1	0
Additional consolidated entities							0	0	0	0	0	1											0	0	0	0	0	0						0	0	0	0	0	0						0	0	0	0	0	1
Deconsolidated entities							0	(1)	0	0	0	(1)											0	0	0	0	0	0						0	0	0	0	0	(1)						0	(1)	0	0	0	0
Entities consolidated at end of period							1	1	2	4	4	4											1	1	1	2	2	2						0	0	0	1	1	1						0	0	1	1	1	1
Number of variable interest entities			2
Assets [Abstract]
Cash and cash equivalents	324,440,000	260,839,000	324,440,000	260,839,000	276,340,000	169,601,000							323,560,000	260,740,000	323,560,000	260,740,000	259,531,000	0	0	0	0	15,000,000							880,000	98,000	880,000	98,000	1,809,000							0	1,000	0	1,000	0
Investments in securities	249,630,000	298,425,000	249,630,000	298,425,000	238,333,000								223,198,000	218,330,000	223,198,000	218,330,000	225,599,000	0	0	0	0	0							6,993,000	7,048,000	6,993,000	7,048,000	6,211,000							19,439,000	73,047,000	19,439,000	73,047,000	6,523,000
Investments in partnerships	102,119,000	98,598,000	102,119,000	98,598,000	100,893,000								109,055,000	160,435,000	109,055,000	160,435,000	107,981,000	1,237,000	1,158,000	1,237,000	1,158,000	933,000							(8,173,000)	(8,892,000)	(8,173,000)	(8,892,000)	(8,021,000)							0	(54,103,000)	0	(54,103,000)	0
Receivable from brokers	41,513,000	35,968,000	41,513,000	35,968,000	20,913,000								25,337,000	11,155,000	25,337,000	11,155,000	17,593,000	0	0	0	0	0							573,000	1,932,000	573,000	1,932,000	270,000							15,603,000	22,881,000	15,603,000	22,881,000	3,050,000
Investment advisory fees receivable													26,028,000	25,801,000	26,028,000	25,801,000	32,157,000	(1,000)	19,000	(1,000)	19,000	1,000							(1,000)	(1,000)	(1,000)	(1,000)	(2,000)							0	(73,000)	0	(73,000)	0
Other assets	23,039,000	25,964,000	23,039,000	25,964,000	28,861,000								23,226,000	26,134,000	23,226,000	26,134,000	43,889,000	9,000	11,000	9,000	11,000	(14,989,000)							0	0	0	0	0							206,000	57,000	206,000	57,000	0
Total assets	826,730,000	810,658,000	826,730,000	810,658,000	756,749,000								789,965,000	767,453,000	789,965,000	767,453,000	745,947,000	(1,245,000)	(1,188,000)	(1,245,000)	(1,188,000)	(945,000)							272,000	207,000	272,000	207,000	284,000							35,248,000	41,810,000	35,248,000	41,810,000	9,573,000
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	(7,010,000)	(10,244,000)	(7,010,000)	(10,244,000)	(5,488,000)								(6,788,000)	(2,910,000)	(6,788,000)	(2,910,000)	(5,488,000)	0	0	0	0	0							0	0	0	0	0							(222,000)	(7,334,000)	(222,000)	(7,334,000)	0
Accrued expenses and other liabilities	(29,641,000)	(34,620,000)	(29,641,000)	(34,620,000)	(24,441,000)								(87,569,000)	(94,928,000)	(87,569,000)	(94,928,000)	(69,929,000)	(56,000)	(117,000)	(56,000)	(117,000)	(51,000)							(20,000)	(31,000)	(20,000)	(31,000)	(28,000)							(10,305,000)	(204,000)	(10,305,000)	(204,000)	(4,652,000)
Debt													(265,598,000)	(260,814,000)	(265,598,000)	(260,814,000)	(263,119,000)	0	0	0	0	0							0	0	0	0	0							0	0	0	0	0
Redeemable noncontrolling interests	(26,162,000)	(35,519,000)	(26,162,000)	(35,519,000)	(6,071,000)								0	0	0	0	0	(1,189,000)	(1,071,000)	(1,189,000)	(1,071,000)	(894,000)							(252,000)	(176,000)	(252,000)	(176,000)	(256,000)							(24,721,000)	(34,272,000)	(24,721,000)	(34,272,000)	(4,921,000)
GAMCO's net interests in consolidated VIEs	21,150,000	55,107,000	21,150,000	55,107,000	1,046,000
Total equity	430,010,000	408,801,000	430,010,000	408,801,000	407,411,000	389,608,000							430,010,000	408,801,000	430,010,000	408,801,000	407,411,000	0	0	0	0	0							0	0	0	0	0							0	0	0	0	0
Total liabilities and equity	826,730,000	810,658,000	826,730,000	810,658,000	756,749,000								(789,965,000)	(767,453,000)	(789,965,000)	(767,453,000)	(745,947,000)	(1,245,000)	(1,188,000)	(1,245,000)	(1,188,000)	(945,000)							(272,000)	(207,000)	(272,000)	(207,000)	(284,000)							(35,248,000)	(41,810,000)	(35,248,000)	(41,810,000)	(9,573,000)
Percentage interest owned by consolidated feeder funds of their offshore master funds (in hundredths)			100.00%
Maximum exposure to loss related to VIEs	8,100,000		8,100,000		5,000,000
Deferred carried interest in VIE	43,000	49,000	43,000	49,000	47,000
Total VIE assets	80,000,000	23,500,000	80,000,000	23,500,000	73,700,000
Condensed Consolidated Statements of Income [Abstract]
Total revenues	81,024,000	85,081,000	162,773,000	161,986,000									80,703,000	85,152,000	163,282,000	162,120,000		(1,000)	0	(2,000)	(4,000)								0	0	(1,000)	(1,000)								322,000	(71,000)	(506,000)	(129,000)
Total expenses	50,657,000	54,584,000	105,394,000	110,729,000									50,649,000	54,432,000	105,170,000	110,406,000		25,000	28,000	48,000	59,000								8,000	8,000	19,000	23,000								(25,000)	116,000	157,000	241,000
Operating income	30,367,000	30,497,000	57,379,000	51,257,000									30,054,000	30,720,000	58,112,000	51,714,000		(26,000)	(28,000)	(50,000)	(63,000)								(8,000)	(8,000)	(20,000)	(24,000)								347,000	(187,000)	(663,000)	(370,000)
Total other income (expense), net	(6,818,000)	2,127,000	3,892,000	9,936,000									(6,361,000)	1,903,000	3,183,000	8,899,000		12,000	(72,000)	97,000	179,000								(6,000)	(160,000)	17,000	(64,000)								(463,000)	456,000	595,000	922,000
Income before income taxes													23,693,000	32,623,000	61,295,000	60,613,000		(14,000)	(100,000)	47,000	116,000								(14,000)	(168,000)	(3,000)	(88,000)								(116,000)	269,000	(68,000)	552,000
Income tax provision	8,686,000	11,945,000	22,442,000	22,233,000									8,686,000	11,945,000	22,442,000	22,233,000		0	0	0	0								0	0	0	0								0	0	0	0
Net income	14,863,000	20,679,000	38,829,000	38,960,000									15,007,000	20,678,000	38,853,000	38,380,000		(14,000)	(100,000)	47,000	116,000								(14,000)	(168,000)	(3,000)	(88,000)								(116,000)	269,000	(68,000)	552,000
Net income/(loss) attributable to noncontrolling interests	(242,000)	32,000	(112,000)	670,000									(98,000)	31,000	(88,000)	90,000		(14,000)	(100,000)	47,000	116,000								(14,000)	(168,000)	(3,000)	(88,000)								(116,000)	269,000	(68,000)	552,000
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 15,105,000	$ 20,647,000	$ 38,941,000	$ 38,290,000									$ 15,105,000	$ 20,647,000	$ 38,941,000	$ 38,290,000		$ 0	$ 0	$ 0	$ 0								$ 0	$ 0	$ 0	$ 0								$ 0	$ 0	$ 0	$ 0