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

Premium above floating interest rate charged on the 2017 Floating Rate Senior Notes.

No definition available.

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

Basis point premium added to the Adjusted LIBO Rate (as defined in the 2017 Term Loan Facility Agreement). The total of which is an option used by the Company to calculate the interest on borrowings depending on the Company's election, depending on the Company's index debt rating.

No definition available.

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

Premium applied to annual interest rate in basis points added to the Alternate Base Rate (as defined in the 2017 Term Loan Facility Agreement).

No definition available.

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

Annual interest rate in base points for Alternate Base Rate Loans as defined in 2015 Facility Agreement.

No definition available.

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

Annual interest rate in basis points for Eurocurrency loans as defined in 2015 Facility Agreement.

No definition available.

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

Amount of principal that if the Company or any of its subsidiaries fails to pay on other borrowings, will result in default under the 2017 Indenture.

No definition available.

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

Minimum percentage of votes needed by holders of the notes which will result in the notes becoming due and payable pursuant to an event of default.

No definition available.

Name:	mco_MinimumPercentageOfAggregatePrincipalAmount
Namespace Prefix:	mco_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of Principal Amount, plus any accrued and unpaid interest up to, but not including, the redemption date on a special mandatory redemption, which the Company must pay in the event that the Company does not complete the Bureau van Dijk acquisition on or prior to January 29, 2018, or the share purchase agreement is terminated at any time prior to such date.

No definition available.

Name:	mco_PercentageOfPrincipalAmountBeingPaidAfterIncompletionOfAcquisitionOrTerminationOfSharePurchaseAgreement
Namespace Prefix:	mco_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of Principal Amount, plus any accrued and unpaid interest to date of purchase which the Company is required to pay upon a "change of Control Triggering Event" as defined in 2017 Indenture.

No definition available.

Name:	mco_PercentageOfPrincipalAmountBeingPaidAfterOccurenceOfTriggeringEvent
Namespace Prefix:	mco_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of principal amount being redeemed, if in whole or in part, including accrued and unpaid interest and a Make-Whole Amount.

No definition available.

Name:	mco_PercentageOfPrincipalAmountBeingPaidAfterRedemptionOfNotes
Namespace Prefix:	mco_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Date the original debt was issued, in CCYY-MM-DD format.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

Name:	us-gaap_DebtConversionOriginalDebtIssuanceDateOfDebtDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
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

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_LineOfCreditFacilityExpirationPeriod
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://www.xbrl.org/2003/role/presentationRef
-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

Weighted average interest rate of short-term debt outstanding.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_ShortTermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

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

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

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

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

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

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

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

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

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