Shareholder-Designated Contributions (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020
Shareholder-Designated Contributions [Abstract]
Expense related to shareholder-designated charitable contribution program	$ 5.4	$ 5.4
Shareholder-designated contribution, diluted per share (in dollars per share)	$ 0.25	$ 0.25

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