CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
ASSETS
Restricted cash	$ 0	$ 0	$ 95
Restricted investments in securities	$ 0	$ 0	$ 59,954
LIABILITIES AND EQUITY
Debt instrument, interest rate		1.60%
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)	29,100,000	29,000,000	29,200,000
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)	15,976,239	15,541,489	15,473,725
Common Stock, outstanding (in shares)	10,041,376	9,949,482	10,075,944
Treasury stock, shares (in shares)	5,934,863	5,592,007	5,397,781
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)	19,024,404	19,024,404	19,092,168
4.5% Convertible Notes [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	4.50%	4.50%	4.50%
Debt issuance costs	$ 0	$ 0	$ 138
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	Feb. 28, 2018
5.875% Senior Notes [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	5.875%	5.875%	5.875%
Debt issuance costs	$ 63	$ 81	$ 87
Debt instrument, maturity date	Jun. 01, 2021

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