CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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
Treasury stock, shares (in shares)	6,012,002	5,592,007
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, shares issued (in shares)	15,969,303	15,541,489
Common stock, shares outstanding (in shares)	9,957,301	9,949,482
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, shares issued (in shares)	24,000,000	24,000,000
Common stock, shares outstanding (in shares)	19,024,240	19,024,404
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%
Debt issuance costs	$ 57	$ 81
Debt instrument, maturity date	Jun. 01, 2021
AC 1.6% Note [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	1.60%
Debt instrument, maturity date	Feb. 28, 2018