Assets And Liabilities Held For Sale - Schedule of Impairement of Held for Sale Assets (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Disposal Group Including Discontinued Operation Impairment [Abstract]
Impairment of held for sale assets	$ 8.7	$ 8.7
MAKS [Member]
Disposal Group Including Discontinued Operation Impairment [Abstract]
Total net assets held for sale prior to impairment	186.9	186.9
Currency translation adjustments, net of tax	26.6	26.6
Total net assets held for sale including currency translation adjustment	213.5	213.5
Estimated fair value less costs to sell	$ 204.8	204.8
Impairment of held for sale assets		$ 8.7

Carrying value of disposal group before impairment loss.

No definition available.

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

- Definition

Net amount classified as assets net of liabilties prior to impairment attributable to disposal group held for sale or disposed of which includes currency translation adjustments.

+ References

No definition available.

+ Details

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

- Definition

Disposal group including discontinued operation impairment charge.

+ References

No definition available.

+ Details

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

- Definition

Increase to the carrying value of the disposal group relating to currency translation adjustments.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of assets classified as held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19190-110258

+ Details

Name:	us-gaap_AssetsHeldForSaleLongLivedFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2611-110228

+ Details

Name:	us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=mco_MaksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: