CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
ASSETS
Investment in GBL stock, shares (in shares)	3,726,250	4,393,055	4,393,055
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, shares outstanding (in shares)	23,100,000	23,600,000	24,200,000
Treasury stock, shares (in shares)	2,459,631	1,952,908	1,343,694
GBL 4% PIK Note [Member]
Equity:
Debt instrument, interest rate	4.00%
Class A [Member]
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, shares issued (in shares)	6,404,287	6,404,287	6,395,380
Common stock, shares outstanding (in shares)	3,944,656	4,451,379	5,051,686
Class B [Member]
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, shares issued (in shares)	19,196,792	19,196,792	19,196,792
Common stock, shares outstanding (in shares)	19,187,885	19,187,885	19,196,792