Shareholder-Designated Charitable Contributions (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Shareholder-Designated Charitable Contributions [Abstract]
Expense related to shareholder-designated charitable contribution program	$ 5.4	$ 4.5

Expense related to the contribution which was included in shareholder-designated charitable contribution program.

No definition available.

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