Investments in Partnerships, Offshore Funds and Variable Interest Entities (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Variable Interest Entity [Line Items]
Investments in affiliated entities	$ 83,900,000				$ 86,900,000				$ 83,900,000	$ 86,900,000
Investments in unaffiliated entities	13,600,000				14,000,000				13,600,000	14,000,000
Assets [Abstract]
Cash and cash equivalents	190,608,000				276,340,000				190,608,000	276,340,000
Investments In Partnerships	97,549,000				100,893,000				97,549,000	100,893,000
Receivable from brokers	50,655,000				20,913,000				50,655,000	20,913,000
Investment advisory fees receivable	42,429,000				32,156,000				42,429,000	32,156,000
Total assets	690,733,000				756,749,000				690,733,000	756,749,000
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	3,136,000				5,488,000				3,136,000	5,488,000
Accrued expenses and other liabilities	26,365,000				24,441,000				26,365,000	24,441,000
Redeemable noncontrolling interests	17,362,000				6,071,000				17,362,000	6,071,000
Total equity	370,934,000				407,411,000				370,934,000	407,411,000	389,608,000	443,661,000
Total liabilities and equity	690,733,000				756,749,000				690,733,000	756,749,000
Maximum exposure to loss related to VIEs	7,700,000				5,000,000				7,700,000	5,000,000
Deferred carried interest in VIE	45,000				47,000				45,000	47,000
Total VIE assets	75,000,000				73,700,000				75,000,000	73,700,000
Condensed Consolidated Statements of Income [Abstract]
Total revenues									344,281,000	327,128,000	280,380,000
Total expenses									233,151,000	213,834,000	189,351,000
Operating income	24,739,000	29,012,000	30,367,000	27,012,000	31,376,000	30,661,000	30,497,000	20,760,000	111,130,000	113,294,000	91,029,000
Total other income (expense), net									6,186,000	(2,852,000)	18,312,000
Income before income taxes									117,316,000	110,442,000	109,341,000
Income tax provision									41,721,000	40,767,000	39,326,000
Net income									75,595,000	69,675,000	70,015,000
Net income/(loss) attributable to noncontrolling interests									56,000	(7,000)	1,223,000
Net income attributable to GAMCO Investors, Inc.'s shareholders									75,539,000	69,682,000	68,792,000
Redemption of proprietary capital from this VIE										49,200,000
Variable Interest Entity, Primary Beneficiary [Member]
Assets [Abstract]
Cash and cash equivalents	0				15,000,000				0	15,000,000
Investments In Partnerships	18,507,000				1,433,000				18,507,000	1,433,000
Receivable from brokers	0				0				0	0
Other assets	0				0				0	0
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0				0				0	0
Accrued expenses and other liabilities	(3,010,000)				(15,006,000)				(3,010,000)	(15,006,000)
Redeemable noncontrolling interests	(411,000)				(381,000)				(411,000)	(381,000)
GAMCO's net interests in consolidated VIEs	15,086,000				1,046,000				15,086,000	1,046,000
VIE [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				1				2	1	2	1
Additional consolidated entities									0	0	1
Deconsolidated entities									0	(1)	0
Entities consolidated at end of period	1				1				1	1	2
VOE [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				4				4	4	4	3
Additional consolidated entities									0	1	1
Deconsolidated entities									0	(1)	0
Entities consolidated at end of period	4				4				4	4	4
As Reported [Member]
Assets [Abstract]
Cash and cash equivalents	190,608,000				276,340,000				190,608,000	276,340,000
Investments in securities	280,715,000				297,547,000				280,715,000	297,547,000
Investments In Partnerships	97,549,000				100,893,000				97,549,000	100,893,000
Receivable from brokers	50,655,000				20,913,000				50,655,000	20,913,000
Investment advisory fees receivable	42,429,000				32,156,000				42,429,000	32,156,000
Other assets	28,777,000				28,900,000				28,777,000	28,900,000
Total assets	690,733,000				756,749,000				690,733,000	756,749,000
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	3,136,000				5,488,000				3,136,000	5,488,000
Accrued expenses and other liabilities	82,935,000				74,660,000				82,935,000	74,660,000
Debt	216,366,000				263,119,000				216,366,000	263,119,000
Redeemable noncontrolling interests	17,362,000				6,071,000				17,362,000	6,071,000
Total equity	370,934,000				407,411,000				370,934,000	407,411,000
Total liabilities and equity	690,733,000				756,749,000				690,733,000	756,749,000
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents	189,743,000				259,531,000				189,743,000	259,531,000
Investments in securities	275,491,000				284,796,000				275,491,000	284,796,000
Investments In Partnerships	100,164,000				107,981,000				100,164,000	107,981,000
Receivable from brokers	25,972,000				17,593,000				25,972,000	17,593,000
Investment advisory fees receivable	42,425,000				32,157,000				42,425,000	32,157,000
Other assets	32,673,000				43,889,000				32,673,000	43,889,000
Total assets	666,468,000				745,947,000				666,468,000	745,947,000
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	3,033,000				5,488,000				3,033,000	5,488,000
Accrued expenses and other liabilities	76,135,000				69,929,000				76,135,000	69,929,000
Debt	216,366,000				263,119,000				216,366,000	263,119,000
Redeemable noncontrolling interests	0				0				0	0
Total equity	370,934,000				407,411,000				370,934,000	407,411,000
Total liabilities and equity	666,468,000				745,947,000				666,468,000	745,947,000
Condensed Consolidated Statements of Income [Abstract]
Total revenues									346,195,000	328,151,000	280,713,000
Total expenses									232,313,000	212,844,000	188,793,000
Operating income									113,882,000	115,307,000	91,920,000
Total other income (expense), net									3,264,000	(4,872,000)	16,054,000
Income before income taxes									117,146,000	110,435,000	107,974,000
Income tax provision									41,721,000	40,767,000	38,817,000
Net income									75,425,000	69,668,000	69,157,000
Net income/(loss) attributable to noncontrolling interests									(114,000)	(14,000)	365,000
Net income attributable to GAMCO Investors, Inc.'s shareholders									75,539,000	69,682,000	68,792,000
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents	0				15,000,000				0	15,000,000
Investments in securities	0				0				0	0
Investments In Partnerships	5,388,000				933,000				5,388,000	933,000
Receivable from brokers	0				0				0	0
Investment advisory fees receivable	9,000				1,000				9,000	1,000
Other assets	(2,986,000)				(14,989,000)				(2,986,000)	(14,989,000)
Total assets	2,411,000				945,000				2,411,000	945,000
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0				0				0	0
Accrued expenses and other liabilities	384,000				51,000				384,000	51,000
Debt	0				0				0	0
Redeemable noncontrolling interests	2,027,000				894,000				2,027,000	894,000
Total equity	0				0				0	0
Total liabilities and equity	2,411,000				945,000				2,411,000	945,000
Condensed Consolidated Statements of Income [Abstract]
Total revenues									2,000	(24,000)	(13,000)
Total expenses									132,000	121,000	51,000
Operating income									(130,000)	(145,000)	(64,000)
Total other income (expense), net									216,000	71,000	264,000
Income before income taxes									86,000	(74,000)	200,000
Income tax provision									0	0	74,000
Net income									86,000	(74,000)	126,000
Net income/(loss) attributable to noncontrolling interests									86,000	(74,000)	126,000
Net income attributable to GAMCO Investors, Inc.'s shareholders									0	0	0
CFFs [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				1				1	1	1	1
Additional consolidated entities									0	0	0
Deconsolidated entities									0	0	0
Entities consolidated at end of period	1				1				1	1	1
CFFs [Member] | VOE [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				2				2	2	2	2
Additional consolidated entities									0	0	0
Deconsolidated entities									0	0	0
Entities consolidated at end of period	2				2				2	2	2
Partnerships [Member]
Assets [Abstract]
Cash and cash equivalents	865,000				1,809,000				865,000	1,809,000
Investments in securities	6,964,000				6,228,000				6,964,000	6,228,000
Investments In Partnerships	(8,003,000)				(8,021,000)				(8,003,000)	(8,021,000)
Receivable from brokers	1,480,000				270,000				1,480,000	270,000
Investment advisory fees receivable	(5,000)				(2,000)				(5,000)	(2,000)
Other assets	(1,000,000)				0				(1,000,000)	0
Total assets	301,000				284,000				301,000	284,000
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0				0				0	0
Accrued expenses and other liabilities	21,000				28,000				21,000	28,000
Debt	0				0				0	0
Redeemable noncontrolling interests	280,000				256,000				280,000	256,000
Total equity	0				0				0	0
Total liabilities and equity	301,000				284,000				301,000	284,000
Condensed Consolidated Statements of Income [Abstract]
Total revenues									(6,000)	(3,000)	(149,000)
Total expenses									39,000	40,000	240,000
Operating income									(45,000)	(43,000)	(389,000)
Total other income (expense), net									67,000	34,000	1,194,000
Income before income taxes									22,000	(9,000)	805,000
Income tax provision									0	0	300,000
Net income									22,000	(9,000)	505,000
Net income/(loss) attributable to noncontrolling interests									22,000	(9,000)	505,000
Net income attributable to GAMCO Investors, Inc.'s shareholders									0	0	0
Partnerships [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				0				0	0	0	0
Additional consolidated entities									0	0	0
Deconsolidated entities									0	0	0
Entities consolidated at end of period	0				0				0	0	0
Partnerships [Member] | VOE [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				1				2	1	2	1
Additional consolidated entities									0	0	1
Deconsolidated entities									0	(1)	0
Entities consolidated at end of period	1				1				1	1	2
Offshore Funds [Member]
Assets [Abstract]
Cash and cash equivalents	0				0				0	0
Investments in securities	(1,740,000)				6,523,000				(1,740,000)	6,523,000
Investments In Partnerships	0				0				0	0
Receivable from brokers	23,203,000				3,050,000				23,203,000	3,050,000
Investment advisory fees receivable	0				0				0	0
Other assets	90,000				0				90,000	0
Total assets	21,553,000				9,573,000				21,553,000	9,573,000
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	103,000				0				103,000	0
Accrued expenses and other liabilities	6,395,000				4,652,000				6,395,000	4,652,000
Debt	0				0				0	0
Redeemable noncontrolling interests	15,055,000				4,921,000				15,055,000	4,921,000
Total equity	0				0				0	0
Total liabilities and equity	21,553,000				9,573,000				21,553,000	9,573,000
Condensed Consolidated Statements of Income [Abstract]
Total revenues									(1,910,000)	(996,000)	(171,000)
Total expenses									667,000	829,000	267,000
Operating income									(2,577,000)	(1,825,000)	(438,000)
Total other income (expense), net									2,639,000	1,915,000	800,000
Income before income taxes									62,000	90,000	362,000
Income tax provision									0	0	135,000
Net income									62,000	90,000	227,000
Net income/(loss) attributable to noncontrolling interests									62,000	90,000	227,000
Net income attributable to GAMCO Investors, Inc.'s shareholders									$ 0	$ 0	$ 0
Offshore Funds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				0				1	0	1	0
Additional consolidated entities									0	0	1
Deconsolidated entities									0	(1)	0
Entities consolidated at end of period	0				0				0	0	1
Offshore Funds [Member] | VOE [Member]
Entities Consolidated [Roll Forward]
Entities consolidated at beginning of period				1				0	1	0	0
Additional consolidated entities									0	1	0
Deconsolidated entities									0	0	0
Entities consolidated at end of period	1				1				1	1	0