Average fixed interest rate related to the group of interest rate derivatives.

No definition available.

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

- Definition

Description of the relevant terms of the derivative. Includes the type of instrument, risk being hedged, notional amount, counterparty, inception date, maturity date, relevant interest rates, strike price, cap price, and floor price.

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

General discussion of the entity's interest rate fair value hedge activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579240-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41641-113959

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41620-113959

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CurrencySwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_NetInvestmentHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mco_FixedRateAxis=mco_FixedReceivedWeightedAverageInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_TransactionTypeAxis=mco_CurrencyPaidMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mco_FixedRateAxis=mco_FixedPayWeightedAverageInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_TransactionTypeAxis=mco_CurrencyReceivedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Instruments And Hedging Activities - Summary of Notional Amounts of Outstanding Cross Currency Swap (Detail) - Cross-currency swap - Net Investment Hedging € in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2020 EUR (€)	Dec. 31, 2019 EUR (€)
3.96%
Derivative [Line Items]
Nature of Swap	Pay Fixed/Receive Fixed	Pay Fixed/Receive Fixed
3-month U.S. LIBOR
Derivative [Line Items]
Nature of Swap	Pay Floating/Receive Floating	Pay Floating/Receive Floating
Currency Paid
Derivative [Line Items]
Notional Amount \| €			€ 2,460	€ 2,010
Currency Paid \| 1.43%
Derivative [Line Items]
Notional Amount \| €			€ 1,079	€ 1,079
Weighted Average Fixed Interest Rate	1.43%	1.43%	1.43%	1.43%
Currency Paid \| 3-month EURIBOR
Derivative [Line Items]
Notional Amount \| €			€ 1,381	€ 931
Weighted Average Floating Interest Rate	Based on 3-month EURIBOR	Based on 3-month EURIBOR
Currency Received
Derivative [Line Items]
Notional Amount \| $	$ 2,800	$ 2,300
Currency Received \| 3.96%
Derivative [Line Items]
Notional Amount \| $	$ 1,220	$ 1,220
Weighted Average Fixed Interest Rate	3.96%	3.96%	3.96%	3.96%
Currency Received \| 3-month U.S. LIBOR
Derivative [Line Items]
Notional Amount \| $	$ 1,580	$ 1,080
Weighted Average Floating Interest Rate	Based on 3-month USD LIBOR	Based on 3-month USD LIBOR