Investments in Partnerships, Offshore Funds and Variable Interest Entities, Part II (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Assets [Abstract]
Cash and cash equivalents	$ 273,353	$ 324,630	$ 190,608	$ 276,340
Investments in securities	229,286	257,607	218,843
Investments in sponsored investment companies	64,294	62,305	61,872
Investments in partnerships	94,260	101,685	97,549
Receivable from brokers	44,583	29,298	50,655
Investment advisory fees receivable	29,624	27,193	42,429
Other asset	25,229	23,707	28,777
Total assets	760,629	826,425	690,733
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	6,377	9,657	3,136
Accrued expense and other liabilities	124,821	111,107	82,935
Total Debt	216,688	264,300	216,366
Redeemable noncontrolling interests	16,414	16,828	17,362
Total equity	396,329	424,533	370,934	407,411
Total liabilities and equity	760,629	826,425	690,733
Condensed Consolidated Statements of Income [Abstract]
Total revenues	86,181	81,749
Total expenses	55,454	54,737
Operating income	30,727	27,012
Total other income, net	5,148	10,710
Income before income taxes	35,875	37,722
Income tax provision	13,195	13,756
Net income	22,680	23,966
Net income/(loss) attributable to noncontrolling interests	135	130
VIE [Member]
Assets [Abstract]
Cash and cash equivalents	21	0	0
Investments in partnerships	15,484	17,183	18,507
Liabilities and Equity [Abstract]
Accrued expense and other liabilities	(1,041)	(7)	(3,010)
Redeemable noncontrolling interests	0	(403)	(411)
GAMCO's net interests in consolidated VIEs	14,464	16,773	15,086
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents	272,454	322,523	189,743
Investments in securities	214,627	236,387	213,639
Investments in sponsored investment companies	64,278	62,289	61,852
Investments in partnerships	99,500	109,136	100,164
Receivable from brokers	30,569	18,400	25,972
Investment advisory fees receivable	29,717	27,189	42,425
Other asset	26,136	23,691	32,673
Total assets	737,281	799,615	666,468
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	5,864	9,633	3,033
Accrued expense and other liabilities	118,401	101,149	76,135
Total Debt	216,688	264,300	216,366
Redeemable noncontrolling interests	(1)	0	0
Total equity	396,329	424,533	370,934
Total liabilities and equity	737,281	799,615	666,468
Condensed Consolidated Statements of Income [Abstract]
Total revenues	86,456	82,579
Total expenses	55,215	54,521
Operating income	31,241	28,058
Total other income, net	4,502	9,544
Income before income taxes	35,743	37,602
Income tax provision	13,195	13,756
Net income	22,548	23,846
Net income/(loss) attributable to noncontrolling interests	3	10
Net income attributable to GAMCO Investors, Inc.'s shareholders	22,545	23,836
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents	534	0	0
Investments in securities	0	0	0
Investments in sponsored investment companies	0	0	0
Investments in partnerships	3,423	990	5,388
Receivable from brokers	0	0	0
Investment advisory fees receivable	(6)	5	9
Other asset	(1,000)	16	(2,986)
Total assets	2,951	1,011	2,411
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0
Accrued expense and other liabilities	614	59	384
Total Debt	0	0	0
Redeemable noncontrolling interests	2,337	952	2,027
Total equity	0	0	0
Total liabilities and equity	2,951	1,011	2,411
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(6)	(1)
Total expenses	53	23
Operating income	(59)	(24)
Total other income, net	110	85
Income before income taxes	51	61
Income tax provision	0	0
Net income	51	61
Net income/(loss) attributable to noncontrolling interests	51	61
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0
Partnerships [Member]
Assets [Abstract]
Cash and cash equivalents	365	2,107	865
Investments in securities	7,733	6,503	6,944
Investments in sponsored investment companies	16	16	20
Investments in partnerships	(8,663)	(8,441)	(8,003)
Receivable from brokers	866	121	1,480
Investment advisory fees receivable	(2)	(1)	(5)
Other asset	0	0	(1,000)
Total assets	315	305	301
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0
Accrued expense and other liabilities	34	38	21
Total Debt	0	0	0
Redeemable noncontrolling interests	281	267	280
Total equity	0	0	0
Total liabilities and equity	315	305	301
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(1)	(1)
Total expenses	10	11
Operating income	(11)	(12)
Total other income, net	15	23
Income before income taxes	4	11
Income tax provision	0	0
Net income	4	11
Net income/(loss) attributable to noncontrolling interests	4	11
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0
Offshore Funds [Member]
Assets [Abstract]
Cash and cash equivalents	0	0	0
Investments in securities	6,926	14,717	(1,740)
Investments in sponsored investment companies	0	0	0
Investments in partnerships	0	0	0
Receivable from brokers	13,148	10,777	23,203
Investment advisory fees receivable	(85)	0	0
Other asset	93	0	90
Total assets	20,082	25,494	21,553
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	513	24	103
Accrued expense and other liabilities	5,772	9,861	6,395
Total Debt	0	0	0
Redeemable noncontrolling interests	13,797	15,609	15,055
Total equity	0	0	0
Total liabilities and equity	20,082	25,494	21,553
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(268)	(828)
Total expenses	176	182
Operating income	(444)	(1,010)
Total other income, net	521	1,058
Income before income taxes	77	48
Income tax provision	0	0
Net income	77	48
Net income/(loss) attributable to noncontrolling interests	77	48
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 0	$ 0