Related Party Transactions (Details) - Chief Executive Officer [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Related party expenses [Abstract]
Compensation	$ (8.5)	$ (3.6)	$ (8.5)	$ (15.8)
Management fee	$ (1.4)	$ (0.6)	$ (1.4)	$ (2.3)

Increase (decrease) in the aggregate amount of expenditures for salaries, wages, profit sharing and incentive compensation, and other employee benefits, including equity-based compensation, and pension and other postretirement benefit expense.

No definition available.

Name:	gbl_IncreaseDecreaseInLaborAndRelatedExpense
Namespace Prefix:	gbl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration