Indebtedness - Fair Value and Carrying Value of Long-Term Debt (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Carrying Amount	$ 6,355	$ 6,422
Estimated Fair Value	7,024	7,256
4.50% 2012 Senior Notes, due 2022
Debt Instrument [Line Items]
Carrying Amount	510	512
Estimated Fair Value	$ 522	$ 530
Notes payable, interest rate	4.50%	4.50%
4.875% 2013 Senior Notes, due 2024
Debt Instrument [Line Items]
Carrying Amount	$ 498	$ 498
Estimated Fair Value	$ 551	$ 562
Notes payable, interest rate	4.875%	4.875%
5.25% 2014 Senior Notes, due 2044
Debt Instrument [Line Items]
Carrying Amount	$ 598	$ 598
Estimated Fair Value	$ 808	$ 828
Notes payable, interest rate	5.25%	5.25%
1.75% 2015 Senior Notes, due 2027
Debt Instrument [Line Items]
Carrying Amount	$ 591	$ 610
Estimated Fair Value	$ 643	$ 674
Notes payable, interest rate	1.75%	1.75%
2.625% 2017 Senior Notes, due 2023
Debt Instrument [Line Items]
Carrying Amount	$ 508	$ 510
Estimated Fair Value	$ 516	$ 522
Notes payable, interest rate	2.625%	2.625%
3.25% 2017 Senior Notes, due 2028
Debt Instrument [Line Items]
Carrying Amount	$ 511	$ 524
Estimated Fair Value	$ 550	$ 561
Notes payable, interest rate	3.25%	3.25%
4.25% 2018 Senior Notes, due 2029
Debt Instrument [Line Items]
Carrying Amount	$ 395	$ 394
Estimated Fair Value	$ 465	$ 480
Notes payable, interest rate	4.25%	4.25%
4.875% 2018 Senior Notes, due 2048
Debt Instrument [Line Items]
Carrying Amount	$ 390	$ 390
Estimated Fair Value	$ 527	$ 544
Notes payable, interest rate	4.875%	4.875%
0.950% Senior Notes Due 2030
Debt Instrument [Line Items]
Carrying Amount	$ 880	$ 909
Estimated Fair Value	$ 919	$ 974
Notes payable, interest rate	0.95%	0.95%
3.75% 2020 Senior Notes, due 2025
Debt Instrument [Line Items]
Carrying Amount	$ 690	$ 693
Estimated Fair Value	$ 768	$ 785
Notes payable, interest rate	3.75%	3.75%
3.25% 2020 Senior Notes, due 2050
Debt Instrument [Line Items]
Carrying Amount	$ 293	$ 293
Estimated Fair Value	$ 313	$ 329
Notes payable, interest rate	3.25%	3.25%
2.55% 2020 Senior Notes, due 2060
Debt Instrument [Line Items]
Carrying Amount	$ 491	$ 491
Estimated Fair Value	$ 442	$ 467
Notes payable, interest rate	2.55%	2.55%

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI http://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI http://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=123466204&loc=SL6031897-161870

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI http://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI http://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

Reference 7: 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(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandTwelveSeniorNotesDueTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwentyThirteenSeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandFourteenSeniorNotesThirtyYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandAndFifteenSeniorNotesDueTwoThousandAndTwentySevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandSeventeenSeniorNotesDueTwoThousandTwentyEightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandEighteenSeniorNoteDueTwoThousandTwentyNineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandEighteenSeniorNoteDueTwoThousandFourtyEightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_A950SeniorNotesDueTwoThousandAndThirtyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandAndTwentySeniorNoteDueTwoThousandAndTwentyFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandAndTwentySeniorNoteDueTwoThousandAndFiftyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=mco_TwoThousandAndTwentySeniorNoteDueTwoThousandAndSixtyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: