Investments in Partnerships, Offshore Funds and Variable Interest Entities, Part II (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Assets [Abstract]
Cash and cash equivalents at end of period	$ 331,634	$ 243,142	$ 298,224	$ 210,451
Investments in securities	158,788	234,414	259,537
Investments in sponsored investment companies	120,831	42,433	39,537
Investments in partnerships	106,943	105,849	107,637
Receivable from brokers	21,615	54,084	76,079
Investment advisory fees receivable	33,703	33,035	41,872
Other asset	26,393	26,863	43,544
Total assets	799,907	739,820	866,430
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	8,569	10,788	10,595
Accrued expense and other liabilities	126,548	131,546	147,543
Total Debt	109,936	112,098	112,163
Redeemable noncontrolling interests	5,519	8,464	68,334
Total equity	549,335	476,924	527,795	460,182
Total liabilities and equity	799,907	739,820	866,430
Condensed Consolidated Statements of Income [Abstract]
Total revenues	103,845	104,477
Total expenses	69,900	67,978
Operating income	33,945	36,499
Total other income, net	5,993	6,093
Income before income taxes	39,938	42,592
Income tax provision	15,179	14,616
Net income	24,759	27,976
Net income/(loss) attributable to noncontrolling interests	(17)	22
Net income attributable to GAMCO Investors, Inc.'s shareholders	24,776	27,954
VIE [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	0	0	0
Investments in partnerships	11,480	13,798	13,434
Liabilities and Equity [Abstract]
Accrued expense and other liabilities	(9)	(16)	(12)
Redeemable noncontrolling interests	(706)	(1,103)	(794)
GAMCO's net interests in consolidated VIEs	10,765	12,679	12,628
Prior to Consolidations [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	331,553	242,826	298,149
Investments in securities	151,028	234,628	200,443
Investments in sponsored investment companies	120,831	42,422	39,537
Investments in partnerships	109,982	110,330	111,380
Receivable from brokers	20,840	39,163	24,301
Investment advisory fees receivable	33,698	33,112	42,102
Other asset	26,377	26,749	43,393
Total assets	794,309	729,230	759,305
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	8,569	10,531	9,991
Accrued expense and other liabilities	126,469	129,677	109,356
Total Debt	109,936	112,098	112,163
Redeemable noncontrolling interests	0	0	0
Total equity	549,335	476,924	527,795
Total liabilities and equity	794,309	729,230	759,305
Condensed Consolidated Statements of Income [Abstract]
Total revenues	103,866	104,701
Total expenses	69,829	67,712
Operating income	34,037	36,989
Total other income, net	5,910	5,541
Income before income taxes	39,947	42,530
Income tax provision	15,179	14,616
Net income	24,768	27,914
Net income/(loss) attributable to noncontrolling interests	(8)	(40)
Net income attributable to GAMCO Investors, Inc.'s shareholders	24,776	27,954
CFFs [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	0	1	(11)
Investments in securities	0	0	0
Investments in sponsored investment companies	0	0	0
Investments in partnerships	5,255	4,636	4,438
Receivable from brokers	0	0	0
Investment advisory fees receivable	6	2	(6)
Other asset	16	8	0
Total assets	5,277	4,647	4,421
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0
Accrued expense and other liabilities	46	88	22
Total Debt	0	0	0
Redeemable noncontrolling interests	5,231	4,559	4,399
Total equity	0	0	0
Total liabilities and equity	5,277	4,647	4,421
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(6)	(7)
Total expenses	33	34
Operating income	(39)	(41)
Total other income, net	50	76
Income before income taxes	11	35
Income tax provision	0	0
Net income	11	35
Net income/(loss) attributable to noncontrolling interests	11	35
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0
Partnerships [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	81	315	86
Investments in securities	7,760	7,708	7,801
Investments in sponsored investment companies	0	11	0
Investments in partnerships	(8,294)	(9,117)	(8,181)
Receivable from brokers	775	1,411	623
Investment advisory fees receivable	(1)	0	(2)
Other asset	0	0	0
Total assets	321	328	327
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	0	0
Accrued expense and other liabilities	33	37	24
Total Debt	0	0	0
Redeemable noncontrolling interests	288	291	303
Total equity	0	0	0
Total liabilities and equity	321	328	327
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(1)	(1)
Total expenses	9	12
Operating income	(10)	(13)
Total other income, net	(1)	4
Income before income taxes	(11)	(9)
Income tax provision	0	0
Net income	(11)	(9)
Net income/(loss) attributable to noncontrolling interests	(11)	(9)
Net income attributable to GAMCO Investors, Inc.'s shareholders	0	0
Offshore Funds [Member]
Assets [Abstract]
Cash and cash equivalents at end of period	0	0	0
Investments in securities	0	(7,922)	51,293
Investments in sponsored investment companies	0	0	0
Investments in partnerships	0	0	0
Receivable from brokers	0	13,510	51,155
Investment advisory fees receivable	0	(79)	(222)
Other asset	0	106	151
Total assets	0	5,615	102,377
Liabilities and Equity [Abstract]
Securities sold, not yet purchased	0	257	604
Accrued expense and other liabilities	0	1,744	38,141
Total Debt	0	0	0
Redeemable noncontrolling interests	0	3,614	63,632
Total equity	0	0	0
Total liabilities and equity	0	5,615	102,377
Condensed Consolidated Statements of Income [Abstract]
Total revenues	(14)	(216)
Total expenses	29	220
Operating income	(43)	(436)
Total other income, net	34	472
Income before income taxes	(9)	36
Income tax provision	0	0
Net income	(9)	36
Net income/(loss) attributable to noncontrolling interests	(9)	36
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 0	$ 0