Financial Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Intersegment Revenue	$ 865.6us-gaap_Revenues	$ 767.2us-gaap_Revenues
Operating, SG&A	465.7	411.1
Adjusted Operating Income	399.9mco_AdjustedOperatingIncome	356.1mco_AdjustedOperatingIncome
Depreciation and amortization	28.6us-gaap_DepreciationAndAmortization	23.1us-gaap_DepreciationAndAmortization
Operating Income	371.3us-gaap_OperatingIncomeLoss	333.0us-gaap_OperatingIncomeLoss
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue	624.6	550.6
Operating, SG&A	281.3	248.6
Adjusted Operating Income	343.3	302.0
Depreciation and amortization	16.0	11.4
Operating Income	327.3	290.6
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue	266.6	241.4
Operating, SG&A	210.0	187.3
Adjusted Operating Income	56.6	54.1
Depreciation and amortization	12.6	11.7
Operating Income	44.0	42.4
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue	(25.6)	(24.8)
Operating, SG&A	(25.6)	(24.8)
Adjusted Operating Income	0	0
Depreciation and amortization	0	0
Operating Income	$ 0	$ 0

operating income before depreciation and amortization

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- Definition

SG&A expenses include such items as compensation and benefits for corporate officers and staff and compensation and other expenses related to sales of products. They also include items such as office rent, business insurance, professional fees and gains and losses from sales and disposal of assets. Operating expenses include costs associated with the development and production of the Company's products and services and their delivery to customers. These expenses principally include employee compensation and benefits and travel costs that are incurred in connection with these activities.

+ References

No definition available.

+ Details

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- Definition

The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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