CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Liabilities and Equity
Zero coupon subordinated debentures, face value	$ 20.5	$ 21.7	$ 21.8
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
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)	14,833,469	14,203,146	13,904,190
Common Stock, outstanding (in shares)	6,592,649	6,121,585	6,685,414
Treasury stock, shares (in shares)	8,240,820	8,081,561	7,218,776
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)	19,424,174	19,624,174	19,920,730