Name:	mco_FairValueOfFinancialInstrumentsLineItems
Namespace Prefix:	mco_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of potential cash payments that could result from the contingent consideration arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

+ Details

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

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

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

Fair Value - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Fair Value Of Financial Instruments [Line Items]
Contingent consideration at fair value	$ 17.2	$ 17.5	$ 13.4	$ 9.0
Minority Interest Ownership Percentage By Noncontrolling Owners	33.00%
Business Acquisition Contingent Consideration Potential Cash Payment	2.5
Other Liabilities
Fair Value Of Financial Instruments [Line Items]
Contingent consideration at fair value	2.3
Accounts Payable And Accrued Liabilities [Member]
Fair Value Of Financial Instruments [Line Items]
Contingent consideration at fair value	$ 14.9