Related Party Transactions (Details) - Chief Executive Officer [Member] $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Related Party Transaction [Line Items]
Compensation	$ (4.9)
Management fee	$ (1.7)

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