Investments in Partnerships, Offshore Funds and Variable Interest Entities, Part II (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Assets [Abstract]
Cash and cash equivalents at end of period	$ 372,284	$ 307,490	$ 372,284	$ 307,490
Investment in Securities, Fair Value	144,008	225,596	144,008	225,596	$ 259,537
Investments in sponsored investment companies	126,305	40,791	126,305	40,791	39,537
Investments in partnerships	108,950	107,896	108,950	107,896	107,637
Receivable from brokers	56,579	58,945	56,579	58,945	76,079
Investment advisory fees receivable	32,766	32,228	32,766	32,228	41,872
Other asset	25,094	31,856	25,094	31,856	43,544
Total assets	865,986	804,802	865,986	804,802	866,430
Liabilities and Equity [Abstract]
Payable to brokers	48,858	19,859	48,858	19,859	43,409
Securities sold, not yet purchased	9,825	14,329	9,825	14,329	10,595
Accrued expense and other liabilities	176,471	158,619	176,471	158,619	147,543
Total Debt	106,628	111,813	106,628	111,813	112,163
Redeemable noncontrolling interests	5,943	22,806	5,943	22,806	68,334
Total equity	567,119	497,235	567,119	497,235	527,795	$ 460,182
Total liabilities and equity	865,986	804,802	865,986	804,802	866,430
Condensed Consolidated Statements of Income [Abstract]
Total revenues	102,704	108,296	206,549	212,773
Total expenses	67,755	70,759	137,655	138,737
Operating income	34,949	37,537	68,894	74,036
Total other income, net	3,221	8,921	9,214	15,014
Income before income taxes	38,170	46,458	78,108	89,050
Income tax provision	14,123	17,135	29,302	31,751
Net income	24,047	29,323	48,806	57,299
Net income/(loss) attributable to noncontrolling interests	(54)	373	(71)	395
Net income attributable to GAMCO Investors, Inc.'s shareholders	24,101	28,950	48,877	56,904
VIE [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	5	0	5	0
Investment in Securities, Fair Value	918	0	918	0	0
Investments in partnerships	11,665	14,125	11,665	14,125	13,434
Receivable from brokers	60	0	60	0	0
Other asset	6	0	6	0	0
Liabilities and Equity [Abstract]
Payable to brokers	(144)	0	(144)	0	0
Securities sold, not yet purchased	(266)	0	(266)	0	0
Accrued expense and other liabilities	(6,027)	(18)	(6,027)	(18)	(12)
Redeemable noncontrolling interests	(812)	(1,155)	(812)	(1,155)	(794)
GAMCO's net interests in consolidated VIEs	5,405	12,952	5,405	12,952	12,628
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	372,204	307,197	372,204	307,197
Investment in Securities, Fair Value	134,555	209,181	134,555	209,181	200,443
Investments in sponsored investment companies	126,305	40,777	126,305	40,777	39,537
Investments in partnerships	107,520	112,305	107,520	112,305	111,380
Receivable from brokers	54,715	38,266	54,715	38,266	24,301
Investment advisory fees receivable	32,719	32,269	32,719	32,269	42,102
Other asset	31,066	31,673	31,066	31,673	43,393
Total assets	859,084	771,668	859,084	771,668	759,305
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	9,413	13,995	9,413	13,995	9,991
Accrued expense and other liabilities	175,924	148,625	175,924	148,625	109,356
Total Debt	106,628	111,813	106,628	111,813	112,163
Redeemable noncontrolling interests	0	0	0	0	0
Total equity	567,119	497,235	567,119	497,235	527,795
Total liabilities and equity	859,084	771,668	859,084	771,668	759,305
Condensed Consolidated Statements of Income [Abstract]
Total revenues	102,715	108,513	206,581	213,214
Total expenses	67,695	70,515	137,524	138,227
Operating income	35,020	37,998	69,057	74,987
Total other income, net	3,167	8,093	9,077	13,634
Income before income taxes	38,187	46,091	78,134	88,621
Income tax provision	14,123	17,135	29,302	31,751
Net income	24,064	28,956	48,832	56,870
Net income/(loss) attributable to noncontrolling interests	(37)	6	(45)	(34)
Net income attributable to GAMCO Investors, Inc.'s shareholders	24,101	28,950	48,877	56,904
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	9	6	9	6
Investment in Securities, Fair Value	0	0	0	0	0
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	11,287	4,962	11,287	4,962	4,438
Receivable from brokers	0	0	0	0	0
Investment advisory fees receivable	5	28	5	28	(6)
Other asset	(5,985)	40	(5,985)	40	0
Total assets	5,316	5,036	5,316	5,036	4,421
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0	0	0
Accrued expense and other liabilities	53	84	53	84	22
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	5,263	4,952	5,263	4,952	4,399
Total equity	0	0	0	0	0
Total liabilities and equity	5,316	5,036	5,316	5,036	4,421
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(10)	(7)	(16)	(14)
Total expenses	39	(20)	72	14
Operating income	(49)	13	(88)	(28)
Total other income, net	41	130	91	206
Income before income taxes	(8)	143	3	178
Income tax provision	0	0	0	0
Net income	(8)	143	3	178
Net income/(loss) attributable to noncontrolling interests	(8)	143	3	178
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0	0	0
Partnerships [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	66	287	66	287
Investment in Securities, Fair Value	8,535	8,602	8,535	8,602	7,801
Investments in sponsored investment companies	0	14	0	14	0
Investments in partnerships	(9,391)	(9,371)	(9,391)	(9,371)	(8,181)
Receivable from brokers	1,804	792	1,804	792	623
Investment advisory fees receivable	32	(2)	32	(2)	(2)
Other asset	8	8	8	8	0
Total assets	1,054	330	1,054	330	327
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	146	0	146	0	0
Accrued expense and other liabilities	326	21	326	21	24
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	582	309	582	309	303
Total equity	0	0	0	0	0
Total liabilities and equity	1,054	330	1,054	330	327
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(1)	(1)	(2)	(2)
Total expenses	21	14	30	26
Operating income	(22)	(15)	(32)	(28)
Total other income, net	19	33	18	37
Income before income taxes	(3)	18	(14)	9
Income tax provision	0	0	0	0
Net income	(3)	18	(14)	9
Net income/(loss) attributable to noncontrolling interests	(3)	18	(14)	9
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0	0	0
Offshore Funds [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	5	0	5	0
Investment in Securities, Fair Value	918	7,813	918	7,813	51,293
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	(466)	0	(466)	0	0
Receivable from brokers	60	19,887	60	19,887	51,155
Investment advisory fees receivable	10	(67)	10	(67)	(222)
Other asset	5	135	5	135	151
Total assets	532	27,768	532	27,768	102,377
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	266	334	266	334	604
Accrued expense and other liabilities	168	9,889	168	9,889	38,141
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	98	17,545	98	17,545	63,632
Total equity	0	0	0	0	0
Total liabilities and equity	532	27,768	532	27,768	$ 102,377
Condensed Consolidated Statements of Income [Abstract]
Total revenues	0	(209)	(14)	(425)
Total expenses	0	250	29	470
Operating income	0	(459)	(43)	(895)
Total other income, net	(6)	665	28	1,137
Income before income taxes	(6)	206	(15)	242
Income tax provision	0	0	0	0
Net income	(6)	206	(15)	242
Net income/(loss) attributable to noncontrolling interests	(6)	206	(15)	242
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 0	$ 0	$ 0	$ 0