CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Liabilities and Equity
Zero coupon subordinated debentures, face value	$ 13.1	$ 13.8	$ 20.5
GAMCO Investors, Inc. stockholders equity
Preferred Stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Preferred Stock, shares authorized (in shares)	10,000,000	10,000,000	10,000,000
Common Stock, outstanding (in shares)	25,900,000	26,100,000	26,000,000
Class A [Member]
GAMCO Investors, Inc. stockholders equity
Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	100,000,000	100,000,000	100,000,000
Common Stock, issued (in shares)	15,230,433	15,012,719	14,833,469
Common Stock, outstanding (in shares)	6,599,982	6,701,930	6,592,649
Treasury stock, shares (in shares)	8,630,451	8,310,789	8,240,820
Class B [Member]
GAMCO Investors, Inc. stockholders equity
Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	100,000,000	100,000,000	100,000,000
Common Stock, issued (in shares)	24,000,000	24,000,000	24,000,000
Common Stock, outstanding (in shares)	19,279,260	19,384,174	19,424,174