Investments in Partnerships, Offshore Funds and Variable Interest Entities, Part II (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Assets [Abstract]
Cash and cash equivalents at end of period	$ 222,776	$ 324,440	$ 222,776	$ 324,440	$ 190,608	$ 276,340
Investments in securities	220,440	249,630	220,440	249,630	218,843
Investments in sponsored investment companies	52,341	59,561	52,341	59,561	61,872
Investments in partnerships	94,535	102,119	94,535	102,119	97,549
Receivable from brokers	39,669	41,513	39,669	41,513	50,655
Investment advisory fees receivable	28,240	26,026	28,240	26,026	42,429
Other asset	25,500	23,441	25,500	23,441	28,777
Total assets	683,501	826,730	683,501	826,730	690,733
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	7,598	7,010	7,598	7,010	3,136
Accrued expense and other liabilities	140,663	97,950	140,663	97,950	82,935
Total Debt	117,028	265,598	117,028	265,598	216,366
Redeemable noncontrolling interests	7,021	26,162	7,021	26,162	17,362
Total equity	411,191	430,010	411,191	430,010	370,934	407,411
Total liabilities and equity	683,501	826,730	683,501	826,730	690,733
Condensed Consolidated Statements of Income [Abstract]
Total revenues	92,290	81,024	178,471	162,773
Total expenses	60,506	50,657	115,960	105,394
Operating income	31,784	30,367	62,511	57,379
Total other income, net	11,852	(6,818)	17,000	3,892
Income before income taxes	43,636	23,549	79,511	61,271
Income tax provision	15,724	8,686	28,919	22,442
Net income	27,912	14,863	50,592	38,829
Net income/(loss) attributable to noncontrolling interests	19	(242)	154	(112)
Net income attributable to GAMCO Investors, Inc.'s shareholders	27,893	15,105	50,438	38,941
VIE [Member]
Assets [Abstract]
Investments in partnerships	13,457	21,831	13,457	21,831	18,507
Liabilities and Equity [Abstract]
Accrued expense and other liabilities	(6)	(12)	(6)	(12)	(3,010)
Redeemable noncontrolling interests	0	(669)	0	(669)	(411)
GAMCO's net interests in consolidated VIEs	13,451	21,150	13,451	21,150	15,086
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	222,441	323,560	222,441	323,560	189,743
Investments in securities	215,755	223,198	215,755	223,198	213,639
Investments in sponsored investment companies	52,332	59,561	52,332	59,561	61,852
Investments in partnerships	100,342	109,055	100,342	109,055	100,164
Receivable from brokers	27,347	25,337	27,347	25,337	25,972
Investment advisory fees receivable	28,326	26,028	28,326	26,028	42,425
Other asset	25,412	23,226	25,412	23,226	32,673
Total assets	671,955	789,965	671,955	789,965	666,468
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	7,427	6,788	7,427	6,788	3,033
Accrued expense and other liabilities	136,309	87,569	136,309	87,569	76,135
Total Debt	117,028	265,598	117,028	265,598	216,366
Redeemable noncontrolling interests	0	0	0	0	0
Total equity	411,191	430,010	411,191	430,010	370,934
Total liabilities and equity	671,955	789,965	671,955	789,965	666,468
Condensed Consolidated Statements of Income [Abstract]
Total revenues	92,613	80,703	179,069	163,282
Total expenses	60,243	50,649	115,458	105,170
Operating income	32,370	30,054	63,611	58,112
Total other income, net	11,219	(6,361)	15,721	3,183
Income before income taxes	43,589	23,693	79,332	61,295
Income tax provision	15,724	8,686	28,919	22,442
Net income	27,865	15,007	50,413	38,853
Net income/(loss) attributable to noncontrolling interests	(28)	(98)	(25)	(88)
Net income attributable to GAMCO Investors, Inc.'s shareholders	27,893	15,105	50,438	38,941
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	0	0	0	0	0
Investments in securities	0	0	0	0	0
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	3,189	1,237	3,189	1,237	5,388
Receivable from brokers	0	0	0	0	0
Investment advisory fees receivable	(10)	(1)	(10)	(1)	9
Other asset	0	9	0	9	(2,986)
Total assets	3,179	1,245	3,179	1,245	2,411
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0	0	0
Accrued expense and other liabilities	59	56	59	56	384
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	3,120	1,189	3,120	1,189	2,027
Total equity	0	0	0	0	0
Total liabilities and equity	3,179	1,245	3,179	1,245	2,411
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(6)	(1)	(12)	(2)
Total expenses	35	25	88	48
Operating income	(41)	(26)	(100)	(50)
Total other income, net	24	12	134	97
Income before income taxes	(17)	(14)	34	47
Income tax provision	0	0	0	0
Net income	(17)	(14)	34	47
Net income/(loss) attributable to noncontrolling interests	(17)	(14)	34	47
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	335	880	335	880	865
Investments in securities	7,292	6,993	7,292	6,993	6,944
Investments in sponsored investment companies	9	0	9	0	20
Investments in partnerships	(8,996)	(8,173)	(8,996)	(8,173)	(8,003)
Receivable from brokers	1,680	573	1,680	573	1,480
Investment advisory fees receivable	0	(1)	0	(1)	(5)
Other asset	0	0	0	0	(1,000)
Total assets	320	272	320	272	301
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0	0	0
Accrued expense and other liabilities	21	20	21	20	21
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	299	252	299	252	280
Total equity	0	0	0	0	0
Total liabilities and equity	320	272	320	272	301
Condensed Consolidated Statements of Income [Abstract]
Total revenues	0	0	(1)	(1)
Total expenses	15	8	25	19
Operating income	(15)	(8)	(26)	(20)
Total other income, net	33	(6)	48	17
Income before income taxes	18	(14)	22	(3)
Income tax provision	0	0	0	0
Net income	18	(14)	22	(3)
Net income/(loss) attributable to noncontrolling interests	18	(14)	22	(3)
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	(2,607)	19,439	(2,607)	19,439	(1,740)
Investments in sponsored investment companies	0	0	0	0	0
Investments in partnerships	0	0	0	0	0
Receivable from brokers	10,642	15,603	10,642	15,603	23,203
Investment advisory fees receivable	(76)	0	(76)	0	0
Other asset	88	206	88	206	90
Total assets	8,047	35,248	8,047	35,248	21,553
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	171	222	171	222	103
Accrued expense and other liabilities	4,274	10,305	4,274	10,305	6,395
Total Debt	0	0	0	0	0
Redeemable noncontrolling interests	3,602	24,721	3,602	24,721	15,055
Total equity	0	0	0	0	0
Total liabilities and equity	8,047	35,248	8,047	35,248	21,553
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(317)	322	(585)	(506)
Total expenses	213	(25)	389	157
Operating income	(530)	347	(974)	(663)
Total other income, net	576	(463)	1,097	595
Income before income taxes	46	(116)	123	(68)
Income tax provision	0	0	0	0
Net income	46	(116)	123	(68)
Net income/(loss) attributable to noncontrolling interests	46	(116)	123	(68)
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 0	$ 0	$ 0	$ 0