CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 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)	5,266,301	5,107,481	4,801,829
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)	15,475,025	15,477,082	15,482,982
Common Stock, outstanding (in shares)	10,208,724	10,369,601	10,681,153
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)	19,092,168	19,093,311	19,093,311
4.5% Convertible Notes [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	4.50%	4.50%	4.50%
Debt issuance costs	$ 147	$ 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
5.875% Senior Notes [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	5.875%	5.875%	5.875%
Debt issuance costs	$ 93	$ 105	$ 116
Debt instrument, maturity date	Jun. 01, 2021