Investments in Partnerships, Offshore Funds and Variable Interest Entities, Part II (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Assets [Abstract]
Cash and cash equivalents	$ 425,616	$ 358,421	$ 425,616	$ 358,421	$ 298,224	$ 210,451
Investment in Securities, Fair Value	126,917	254,630	126,917	254,630	259,537
Investments in sponsored investment companies	115,046	39,520	115,046	39,520	39,537
Investments in partnerships	101,025	107,434	101,025	107,434	107,637
Receivable from brokers	52,918	79,885	52,918	79,885	76,079
Investment advisory fees receivable	28,629	31,155	28,629	31,155	41,872
Other asset	22,518	27,715	22,518	27,715	43,544
Total assets	872,669	898,760	872,669	898,760	866,430
Liabilities and Equity [Abstract]
Payable to brokers	49,074	46,790	49,074	46,790	43,409
Securities sold, not yet purchased	5,577	14,180	5,577	14,180	10,595
Accrued expense and other liabilities	189,044	205,320	189,044	205,320	147,543
Total Debt	106,750	111,941	106,750	111,941	112,163
Redeemable noncontrolling interests	6,018	56,086	6,018	56,086	68,334
Total equity	565,280	511,233	565,280	511,233	527,795	$ 460,182
Total liabilities and equity	872,669	898,760	872,669	898,760	866,430
Condensed Consolidated Statements of Income [Abstract]
Total revenues	96,546	110,858	303,095	323,631
Total expenses	60,350	67,272	198,005	206,009
Operating income	36,196	43,586	105,090	117,622
Total other income, net	(12,500)	(9,989)	(3,286)	5,025
Income before income taxes	23,696	33,597	101,804	122,647
Income tax provision	9,245	13,045	38,547	44,796
Net income	14,451	20,552	63,257	77,851
Net income/(loss) attributable to noncontrolling interests	(518)	(3,113)	(589)	(2,718)
Net income attributable to GAMCO Investors, Inc.'s shareholders	14,969	23,665	63,846	80,569
VIE [Member]
Assets [Abstract]
Cash and cash equivalents	13	1	13	1	0
Investment in Securities, Fair Value	709	0	709	0	0
Investments in partnerships	5,116	13,618	5,116	13,618	13,434
Receivable from brokers	25	0	25	0	0
Other asset	3	0	3	0	0
Liabilities and Equity [Abstract]
Payable to brokers	(62)	0	(62)	0	0
Securities sold, not yet purchased	(206)	0	(206)	0	0
Accrued expense and other liabilities	(27)	(15)	(27)	(15)	(12)
Redeemable noncontrolling interests	(615)	(962)	(615)	(962)	(794)
GAMCO's net interests in consolidated VIEs	4,956	12,642	4,956	12,642	12,628
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents	425,549	358,316	425,549	358,316	298,149
Investment in Securities, Fair Value	118,401	203,280	118,401	203,280	200,443
Investments in sponsored investment companies	115,046	39,520	115,046	39,520	39,537
Investments in partnerships	105,097	110,679	105,097	110,679	111,380
Receivable from brokers	50,960	31,388	50,960	31,388	24,301
Investment advisory fees receivable	28,615	31,221	28,615	31,221	42,102
Other asset	22,363	28,528	22,363	28,528	43,393
Total assets	866,031	802,932	866,031	802,932	759,305
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	5,231	13,549	5,231	13,549	9,991
Accrued expense and other liabilities	188,770	166,209	188,770	166,209	109,356
Total Debt	106,750	111,941	106,750	111,941	112,163
Redeemable noncontrolling interests	0	0	0	0	0
Total equity	565,280	511,233	565,280	511,233	527,795
Total liabilities and equity	866,031	802,932	866,031	802,932	759,305
Condensed Consolidated Statements of Income [Abstract]
Total revenues	96,558	111,073	303,139	324,287
Total expenses	60,304	66,980	197,828	205,207
Operating income	36,254	44,093	105,311	119,080
Total other income, net	(12,093)	(7,462)	(3,016)	6,172
Income before income taxes	24,161	36,631	102,295	125,252
Income tax provision	9,245	13,045	38,547	44,796
Net income	14,916	23,586	63,748	80,456
Net income/(loss) attributable to noncontrolling interests	(53)	(79)	(98)	(113)
Net income attributable to GAMCO Investors, Inc.'s shareholders	14,969	23,665	63,846	80,569
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents	2	7	2	7	(11)
Investment in Securities, Fair Value	0	0	0	0	0
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	4,817	4,684	4,817	4,684	4,438
Receivable from brokers	0	0	0	0	0
Investment advisory fees receivable	5	17	5	17	(6)
Other asset	16	24	16	24	0
Total assets	4,840	4,732	4,840	4,732	4,421
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0	0	0
Accrued expense and other liabilities	38	71	38	71	22
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	4,802	4,661	4,802	4,661	4,399
Total equity	0	0	0	0	0
Total liabilities and equity	4,840	4,732	4,840	4,732	4,421
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(10)	(7)	(26)	(21)
Total expenses	23	20	95	34
Operating income	(33)	(27)	(121)	(55)
Total other income, net	(268)	(186)	(177)	20
Income before income taxes	(301)	(213)	(298)	(35)
Income tax provision	0	0	0	0
Net income	(301)	(213)	(298)	(35)
Net income/(loss) attributable to noncontrolling interests	(301)	(213)	(298)	(35)
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0	0	0
Partnerships [Member]
Assets [Abstract]
Cash and cash equivalents	52	98	52	98	86
Investment in Securities, Fair Value	7,807	8,836	7,807	8,836	7,801
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	(8,504)	(7,929)	(8,504)	(7,929)	(8,181)
Receivable from brokers	1,933	306	1,933	306	623
Investment advisory fees receivable	9	(1)	9	(1)	(2)
Other asset	137	(1,000)	137	(1,000)	0
Total assets	1,434	310	1,434	310	327
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	140	0	140	0	0
Accrued expense and other liabilities	155	31	155	31	24
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	1,139	279	1,139	279	303
Total equity	0	0	0	0	0
Total liabilities and equity	1,434	310	1,434	310	327
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(2)	0	(4)	(2)
Total expenses	20	12	50	38
Operating income	(22)	(12)	(54)	(40)
Total other income, net	(120)	(18)	(102)	19
Income before income taxes	(142)	(30)	(156)	(21)
Income tax provision	0	0	0	0
Net income	(142)	(30)	(156)	(21)
Net income/(loss) attributable to noncontrolling interests	(142)	(30)	(156)	(21)
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0	0	0
Offshore Funds [Member]
Assets [Abstract]
Cash and cash equivalents	13	0	13	0	0
Investment in Securities, Fair Value	709	42,514	709	42,514	51,293
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	(385)	0	(385)	0	0
Receivable from brokers	25	48,191	25	48,191	51,155
Investment advisory fees receivable	0	(82)	0	(82)	(222)
Other asset	2	163	2	163	151
Total assets	364	90,786	364	90,786	102,377
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	206	631	206	631	604
Accrued expense and other liabilities	81	39,009	81	39,009	38,141
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	77	51,146	77	51,146	63,632
Total equity	0	0	0	0	0
Total liabilities and equity	364	90,786	364	90,786	$ 102,377
Condensed Consolidated Statements of Income [Abstract]
Total revenues	0	(208)	(14)	(633)
Total expenses	3	260	32	730
Operating income	(3)	(468)	(46)	(1,363)
Total other income, net	(19)	(2,323)	9	(1,186)
Income before income taxes	(22)	(2,791)	(37)	(2,549)
Income tax provision	0	0	0	0
Net income	(22)	(2,791)	(37)	(2,549)
Net income/(loss) attributable to noncontrolling interests	(22)	(2,791)	(37)	(2,549)
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 0	$ 0	$ 0	$ 0