Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information [Abstract]
Revenues	$ 116,751us-gaap_SalesRevenueNet	$ 110,858us-gaap_SalesRevenueNet	$ 108,296us-gaap_SalesRevenueNet	$ 104,477us-gaap_SalesRevenueNet	$ 122,714us-gaap_SalesRevenueNet	$ 96,377us-gaap_SalesRevenueNet	$ 92,290us-gaap_SalesRevenueNet	$ 86,181us-gaap_SalesRevenueNet	$ 440,382us-gaap_SalesRevenueNet	$ 397,562us-gaap_SalesRevenueNet
Operating income	41,904us-gaap_OperatingIncomeLoss	43,586us-gaap_OperatingIncomeLoss	37,537us-gaap_OperatingIncomeLoss	36,499us-gaap_OperatingIncomeLoss	47,173us-gaap_OperatingIncomeLoss	32,728us-gaap_OperatingIncomeLoss	31,784us-gaap_OperatingIncomeLoss	30,727us-gaap_OperatingIncomeLoss	159,526us-gaap_OperatingIncomeLoss	142,412us-gaap_OperatingIncomeLoss	111,130us-gaap_OperatingIncomeLoss
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 28,821	$ 23,665	$ 28,950	$ 27,954	$ 33,317	$ 33,098	$ 27,893	$ 22,545	$ 109,390	$ 116,853
Net income attributable to GAMCO Investors, Inc.'s shareholders per share [Abstract]
Basic (in dollars per share)	$ 1.14	$ 0.94	$ 1.14	$ 1.10	$ 1.30	$ 1.29	$ 1.09	$ 0.88	$ 4.32	$ 4.56
Diluted (in dollars per share)	$ 1.13	$ 0.93	$ 1.13	$ 1.09	$ 1.29	$ 1.29	$ 1.09	$ 0.88	$ 4.28	$ 4.54

The amount of net income (loss) from continuing operations per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) derived from continuing operations during the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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- Definition

Amount of income (loss) from a disposal group, net of income tax before extraordinary items allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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