CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
GAMCO Investors, Inc. stockholders equity
Preferred stock, par value (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 Stock, outstanding (in shares)	27,700,000	27,400,000
Treasury stock, shares (in shares)	7,901,529	7,846,436
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)	16,582,676	16,202,726
Common Stock, outstanding (in shares)	8,681,147	8,356,290
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)	19,024,117	19,024,117
5.875% Senior Notes [Member]
LIABILITIES AND STOCKHOLDERS' EQUITY
Debt issuance costs	$ 28	$ 34
Debt instrument, maturity date	Jun. 01, 2021