CONDENSED CONSOLIDATED STATEMENTS OF INCOME UNAUDITED (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Condensed Consolidated Statements of Income [Abstract]
Launch of a new closed-end fund	$ 0	$ 0	$ 0	$ 5.6

Costs directly related to the launch of a new closed-end fund.

No definition available.

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