CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
LIABILITIES AND EQUITY
Zero coupon subordinated debentures, face value	$ 21.7	$ 86.3
GAMCO Investors, Inc. stockholders equity
Preferred Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized (in shares)	10,000,000	10,000,000
Preferred Stock, Shares Issued (in shares)	0	0
Preferred Stock, Shares Outstanding (in shares)	0	0
Common Class A [Member]
GAMCO Investors, Inc. stockholders equity
Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	100,000,000	100,000,000
Common Stock, issued (in shares)	14,203,146	13,627,397
Common Stock, outstanding (in shares)	6,121,585	6,684,149
Treasury stock, shares (in shares)	8,081,561	6,943,248
Common Class B [Member]
GAMCO Investors, Inc. stockholders equity
Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	100,000,000	100,000,000
Common Stock, issued (in shares)	24,000,000	24,000,000
Common Stock, outstanding (in shares)	19,624,174	20,070,746