Revenues - Schedule of Changes in the Deferred Revenue Balances (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Change In Contract with Customer Liability [Abstract]
Total Changes in deferred revenue			$ (8.1)	$ 55.3
Deferred revenue - current	$ 952.3	$ 860.5	952.3	860.5	$ 953.4
Deferred revenue - noncurrent	118.5	126.0	118.5	126.0	$ 122.3
Contract with Customer Liability Roll forward [Member]
Change In Contract with Customer Liability [Abstract]
Beginning Balance	1,183.2	1,126.5	1,075.7	946.3
Revenue recognized that was included in the contract liability balance at the beginning of the period	(454.0)	(432.0)	(694.1)	(593.1)
Increases due to billings excluding amounts recognized as revenue during the period	346.9	319.5	689.3	648.3
Amount included in liabilities reclassified as held for sale	(2.7)		(2.7)
Effect of exchange rate changes	(2.6)	(27.5)	2.6	(15.0)
Total Changes in deferred revenue	(112.4)	(140.0)	(4.9)	40.2
Ending Balance	1,070.8	986.5	1,070.8	986.5
MIS [Member]
Change In Contract with Customer Liability [Abstract]
Deferred revenue - current	261.7	255.9	261.7	255.9
Deferred revenue - noncurrent	114.3	121.6	114.3	121.6
MIS [Member] \| Contract with Customer Liability Roll forward [Member]
Change In Contract with Customer Liability [Abstract]
Beginning Balance	388.3	397.5	325.4	334.7
Revenue recognized that was included in the contract liability balance at the beginning of the period	(113.6)	(127.2)	(160.5)	(165.4)
Increases due to billings excluding amounts recognized as revenue during the period	100.9	112.6	210.1	212.6
Effect of exchange rate changes	0.4	(5.4)	1.0	(4.4)
Total Changes in deferred revenue	(12.3)	(20.0)	50.6	42.8
Ending Balance	376.0	377.5	376.0	377.5
MA [Member]
Change In Contract with Customer Liability [Abstract]
Deferred revenue - current	690.6	604.6	690.6	604.6
Deferred revenue - noncurrent	4.2	4.4	4.2	4.4
MA [Member] \| Contract with Customer Liability Roll forward [Member]
Change In Contract with Customer Liability [Abstract]
Beginning Balance	794.9	729.0	750.3	611.6
Revenue recognized that was included in the contract liability balance at the beginning of the period	(340.4)	(304.8)	(533.6)	(427.7)
Increases due to billings excluding amounts recognized as revenue during the period	246.0	206.9	479.2	435.7
Amount included in liabilities reclassified as held for sale	(2.7)		(2.7)
Effect of exchange rate changes	(3.0)	(22.1)	1.6	(10.6)
Total Changes in deferred revenue	(100.1)	(120.0)	(55.5)	(2.6)
Ending Balance	$ 694.8	$ 609.0	$ 694.8	$ 609.0

Amount reclassified as liabilities attributable to disposal group held for sale or disposed of.

No definition available.

Name:	mco_DisposalGroupIncludingDiscontinuedOperationLiabilitiesReclassifiedAsHeldForSale
Namespace Prefix:	mco_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) from the effect of exchange rate changes on deferred revenue balances.

+ References

No definition available.

+ Details

Name:	mco_EffectOfExchangeRateOnDeferredRevenue
Namespace Prefix:	mco_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130532-203044

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

+ Details

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

- Definition

Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in measure of progress which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130551-203045

+ Details

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

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130532-203044

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

+ Details

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

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as noncurrent.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130532-203044

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

+ Details

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

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

+ Details

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

- Definition

Amount of increase (decrease) in obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

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

- Details

Name:	srt_ProductOrServiceAxis=mco_ContractWithCustomerLiabilityRollForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mco_MoodysInvestorsServiceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mco_MoodysAnalyticsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: