Contingencies - Additional Information (Detail) $ in Millions	Mar. 31, 2019 USD ($)
Contingencies [Abstract]
Unsupported Allegations of Minimum Underpaid Taxes	$ 120.0

Purported underpayment of taxes by plaintiff which the Company believes is unsupported as a matter of fact and law.

No definition available.

Name:	mco_UnsupportedAllegationsOfMinimumUnderpaidTaxes
Namespace Prefix:	mco_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant