CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
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
Preferred stock, shares issued (in shares)	0	0	0
Preferred stock, shares outstanding (in shares)	0	0	0
Common Stock, outstanding (in shares)	29,300,000	29,500,000	29,800,000
Treasury stock, shares (in shares)	5,232,891	5,107,481	4,789,297
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)	15,474,992	15,477,082	15,472,901
Common Stock, outstanding (in shares)	10,242,101	10,369,601	10,683,604
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)	19,092,201	19,093,311	19,106,792
4.5% Convertible Notes [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	4.50%	4.50%	4.50%
Debt issuance costs	$ 156	$ 165	$ 0
Debt instrument, maturity date	Aug. 15, 2021
AC 4% PIK Note [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	4.00%
Debt instrument, maturity date	Nov. 30, 2020
Loan from GGCP [Member]
LIABILITIES AND EQUITY
Debt instrument, maturity date	Dec. 28, 2016
5.875% Senior Notes [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	5.875%	5.875%	5.875%
Debt issuance costs	$ 99	$ 105	$ 122
Debt instrument, maturity date	Jun. 01, 2021