Name:	mco_DebtInstrumentFacilityFeeBasisPoints
Namespace Prefix:	mco_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Maximum Debt to EBITDA ratio to be maintained by the company debt facility.

No definition available.

Name:	mco_DebtInstrumentMaximumLeverageRatio
Namespace Prefix:	mco_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

Amount of liability recognized arising from contingent consideration in a business combination.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 35
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116859824&loc=d3e6819-128478

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

Name:	us-gaap_BusinessCombinationContingentConsiderationLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of cash outflow for cost from early extinguishment and prepayment of debt. Includes, but is not limited to, third-party cost, premium paid, and other fee paid to lender directly for debt extinguishment or debt prepayment. Excludes accrued interest.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

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

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

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

Name:	us-gaap_DebtInstrumentAxis=mco_TwoThousandEighteenCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CreditFacilityAxis=mco_TermLoanFacilityAnyFiscalQuarterMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CreditFacilityAxis=mco_TermLoanFacilityFirstThreeConsecutiveQuartersImmediatelyFollowingAnyAcquisitionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DebtInstrumentAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DebtInstrumentAxis=mco_TwentyTenSeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DebtInstrumentAxis=mco_TwoThousandFourteenSeniorNotesFiveYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DebtInstrumentAxis=mco_TwoThousandNineteenSeniorNoteDueTwoThousandThirtyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_VariableRateAxis=mco_AlternateBaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: