Fair Value - Additional Information (Detail)	Jun. 30, 2018 USD ($)
Fair Value Inputs [Abstract]
Net asset stable value for public traded funds	$ 1

The money market mutual funds publicly traded funds stable net asset value.

No definition available.

Name:	mco_StableNetAssetValuePublicTradedFunds
Namespace Prefix:	mco_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant