Investments in Partnerships, Offshore Funds and Variable Interest Entities, Part II (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Assets [Abstract]
Cash and cash equivalents at end of period	$ 245,411	$ 288,685	$ 245,411	$ 288,685	$ 190,608	$ 276,340
Investments in securities	237,744	235,445	237,744	235,445	218,843
Investments in sponsored investment companies	43,688	64,223	43,688	64,223	61,872
Investments in partnerships	97,752	102,604	97,752	102,604	97,549
Receivable from brokers	43,854	55,159	43,854	55,159	50,655
Investment advisory fees receivable	31,151	29,187	31,151	29,187	42,429
Other asset	35,022	23,268	35,022	23,268	28,777
Total assets	734,622	798,571	734,622	798,571	690,733
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	7,725	3,856	7,725	3,856	3,136
Accrued expense and other liabilities	165,139	114,995	165,139	114,995	82,935
Total Debt	117,347	216,118	117,347	216,118	216,366
Redeemable noncontrolling interests	5,765	20,228	5,765	20,228	17,362
Total equity	438,646	443,374	438,646	443,374	370,934	407,411
Total liabilities and equity	734,622	798,571	734,622	798,571	690,733
Condensed Consolidated Statements of Income [Abstract]
Total revenues	96,377	82,231	274,848	245,004
Total expenses	63,649	53,219	179,609	158,613
Operating income	32,728	29,012	95,239	86,391
Total other income, net	17,991	(1,446)	34,991	2,446
Income before income taxes	50,719	27,566	130,230	88,837
Income tax provision	17,515	8,467	46,434	30,909
Net income	33,204	19,099	83,796	57,928
Net income/(loss) attributable to noncontrolling interests	106	95	260	(17)
Net income attributable to GAMCO Investors, Inc.'s shareholders	33,098	19,004	83,536	57,945
VIE [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	1,082	0	1,082	0	0
Investments in partnerships	13,782	23,086	13,782	23,086	18,507
Liabilities and Equity [Abstract]
Accrued expense and other liabilities	(1,088)	(15)	(1,088)	(15)	(3,010)
Redeemable noncontrolling interests	0	(962)	0	(962)	(411)
GAMCO's net interests in consolidated VIEs	13,776	22,109	13,776	22,109	15,086
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	243,995	287,806	243,995	287,806	189,743
Investments in securities	240,113	222,489	240,113	222,489	213,639
Investments in sponsored investment companies	43,677	64,223	43,677	64,223	61,852
Investments in partnerships	104,010	109,801	104,010	109,801	100,164
Receivable from brokers	26,981	27,597	26,981	27,597	25,972
Investment advisory fees receivable	31,241	29,182	31,241	29,182	42,425
Other asset	34,947	23,047	34,947	23,047	32,673
Total assets	724,964	764,145	724,964	764,145	666,468
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	7,577	3,879	7,577	3,879	3,033
Accrued expense and other liabilities	161,394	100,774	161,394	100,774	76,135
Total Debt	117,347	216,118	117,347	216,118	216,366
Redeemable noncontrolling interests	0	0	0	0	0
Total equity	438,646	443,374	438,646	443,374	370,934
Total liabilities and equity	724,964	764,145	724,964	764,145	666,468
Condensed Consolidated Statements of Income [Abstract]
Total revenues	96,620	82,489	275,689	245,771
Total expenses	63,400	52,976	178,858	158,146
Operating income	33,220	29,513	96,831	87,625
Total other income, net	17,404	(2,032)	33,125	1,151
Income before income taxes	50,624	27,481	129,956	88,776
Income tax provision	17,515	8,467	46,434	30,909
Net income	33,109	19,014	83,522	57,867
Net income/(loss) attributable to noncontrolling interests	11	10	(14)	(78)
Net income attributable to GAMCO Investors, Inc.'s shareholders	33,098	19,004	83,536	57,945
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	1,082	0	1,082	0	0
Investments in securities	0	0	0	0	0
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	3,253	1,540	3,253	1,540	5,388
Receivable from brokers	0	0	0	0	0
Investment advisory fees receivable	(8)	6	(8)	6	9
Other asset	0	9	0	9	(2,986)
Total assets	4,327	1,555	4,327	1,555	2,411
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0	0	0
Accrued expense and other liabilities	1,146	68	1,146	68	384
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	3,181	1,487	3,181	1,487	2,027
Total equity	0	0	0	0	0
Total liabilities and equity	4,327	1,555	4,327	1,555	2,411
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(8)	(2)	(20)	(4)
Total expenses	46	28	134	76
Operating income	(54)	(30)	(154)	(80)
Total other income, net	94	78	228	175
Income before income taxes	40	48	74	95
Income tax provision	0	0	0	0
Net income	40	48	74	95
Net income/(loss) attributable to noncontrolling interests	40	48	74	95
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	334	879	334	879	865
Investments in securities	7,400	6,908	7,400	6,908	6,944
Investments in sponsored investment companies	11	0	11	0	20
Investments in partnerships	(9,511)	(8,737)	(9,511)	(8,737)	(8,003)
Receivable from brokers	2,115	1,255	2,115	1,255	1,480
Investment advisory fees receivable	(1)	(1)	(1)	(1)	(5)
Other asset	0	0	0	0	(1,000)
Total assets	348	304	348	304	301
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0	0	0
Accrued expense and other liabilities	32	30	32	30	21
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	316	274	316	274	280
Total equity	0	0	0	0	0
Total liabilities and equity	348	304	348	304	301
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(1)	(1)	(2)	(2)
Total expenses	11	11	36	30
Operating income	(12)	(12)	(38)	(32)
Total other income, net	31	34	79	51
Income before income taxes	19	22	41	19
Income tax provision	0	0	0	0
Net income	19	22	41	19
Net income/(loss) attributable to noncontrolling interests	19	22	41	19
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	0	0	0	0	0
Investments in securities	(9,769)	6,048	(9,769)	6,048	(1,740)
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	0	0	0	0	0
Receivable from brokers	14,758	26,307	14,758	26,307	23,203
Investment advisory fees receivable	(81)	0	(81)	0	0
Other asset	75	212	75	212	90
Total assets	4,983	32,567	4,983	32,567	21,553
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	148	(23)	148	(23)	103
Accrued expense and other liabilities	2,567	14,123	2,567	14,123	6,395
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	2,268	18,467	2,268	18,467	15,055
Total equity	0	0	0	0	0
Total liabilities and equity	4,983	32,567	4,983	32,567	21,553
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(234)	(255)	(819)	(761)
Total expenses	192	204	581	361
Operating income	(426)	(459)	(1,400)	(1,122)
Total other income, net	462	474	1,559	1,069
Income before income taxes	36	15	159	(53)
Income tax provision	0	0	0	0
Net income	36	15	159	(53)
Net income/(loss) attributable to noncontrolling interests	36	15	159	(53)
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 0	$ 0	$ 0	$ 0