CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
LIABILITIES AND EQUITY
Zero coupon subordinated debentures, face value	$ 86.3	$ 86.3	$ 86.4
GAMCO Investors, Inc. stockholders equity
Treasury stock, shares (in shares)	7,167,981	6,943,248	6,653,870
Common 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)	13,760,697	13,627,397	13,256,203
Common Stock, outstanding (in shares)	6,592,716	6,684,149	6,872,333
Common 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)	20,040,746	20,070,746	20,190,140