Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information [Abstract]
Revenues	$ 122,714	$ 96,377	$ 92,290	$ 86,181	$ 99,277	$ 82,231	$ 81,024	$ 81,749	$ 397,562	$ 344,281
Operating income	47,173	32,728	31,784	30,727	24,739	29,012	30,367	27,012	142,412	111,130	113,294
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 33,317	$ 33,098	$ 27,893	$ 22,545	$ 17,594	$ 19,004	$ 15,105	$ 23,836	$ 116,853	$ 75,539
Net income attributable to GAMCO Investors, Inc.'s shareholders per share [Abstract]
Basic (in dollars per share)	$ 1.30	$ 1.29	$ 1.09	$ 0.88	$ 0.67	$ 0.72	$ 0.58	$ 0.90	$ 4.56	$ 2.87
Diluted (in dollars per share)	$ 1.29	$ 1.29	$ 1.09	$ 0.88	$ 0.67	$ 0.72	$ 0.57	$ 0.90	$ 4.54	$ 2.86