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

- Definition

Charge recorded in the fourth quarter of 2016 related to an agreement entered into on January 13, 2017 with the U.S. Department of Justice and the attorneys general of 21 U.S. states and the District of Columbia to resolve pending and potential civil claims related to credit ratings that MIS assigned to certain structured finance instruments in the financial crisis era.

+ References

No definition available.

+ Details

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

- Definition

Quarterly Financial Data [Line Items]

+ References

No definition available.

+ Details

Name:	mco_QuarterlyFinancialDataLineItems
Namespace Prefix:	mco_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of foreign currency translation gain (loss) in the disposal group, including discontinued operation, recognized in the statement of income as a result of the sale or complete or substantially complete liquidation of an investment in a foreign entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section 45
-Paragraph 20
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64808268&loc=d3e32211-110900

+ Details

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationForeignCurrencyTranslationGainsLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of judgment or settlement awarded to (against) the entity in respect of litigation.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_EffectOfFourthQuarterEventsByTypeAxis=mco_AfterTaxMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Quarterly Financial Data - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016
Quarterly Financial Data [Line Items]
Benefit to net income related to the resolution of legacy tax matters		$ 1.6	$ 6.4
Accrued settlement charge	$ 863.8			$ 863.8
FX gain relating to the liquidation of a subsidiary	35.0			$ 36.6
Settlement charges	863.8
After Tax [Member]
Quarterly Financial Data [Line Items]
Settlement charges	$ 700.7