Description of Business and Basis of Presentation - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments	2
Excess tax benefit effect of adoption adjustment on provision for income taxes	$ 39.5

Amount of benefit to the income tax provision, as an effect of the adoption of ASU 2016-09, which caused the Excess Tax Benefits or shortfalls recognized on stock-based compensation expense to now be reflected in the consolidated statement of operations as a component of the provision for income taxes.

No definition available.

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