Quarterly Financial Data - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
Sep. 30, 2015	Sep. 30, 2014
Quarterly Financial Data [Line Items]
Benefit to net income related to the resolution of legacy tax matters	$ 6.4	$ 6.4

Benefit to net income related to the resolution of Legacy Tax Matters.

No definition available.

Name:	mco_IncomeBenefitsRelatedToResolutionOfLegacyTaxMatters
Namespace Prefix:	mco_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration