Revenue by Line of Business within Each Reportable Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$ 731.8	$ 646.8
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Total external revenue	521.2	452.7
Intersegment royalty	18.9	17.1
Moodys Investors Service [Member] \| Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenue	258.3	200.5
Moodys Investors Service [Member] \| Structured Finance S F G [Member]
Segment Reporting Information [Line Items]
Revenue	93.0	94.3
Moodys Investors Service [Member] \| Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenue	86.5	78.8
Moodys Investors Service [Member] \| Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenue	83.4	79.1
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Total external revenue	210.6	194.1
Intersegment royalty	2.8	3.0
Moodys Analytics [Member] \| Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenue	129.6	119.8
Moodys Analytics [Member] \| Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenue	53.0	48.1
Moodys Analytics [Member] \| Professional Services [Member]
Segment Reporting Information [Line Items]
Revenue	28.0	26.2
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenue	$ (21.7)	$ (20.1)

Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.

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=6880815&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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- Definition

Amount of revenue from external customers for the reportable segment. Such disclosure is presented if the amount is: (a) included in the measure of segment profit or loss reviewed by the chief operating decision maker or (b) otherwise regularly provided to the chief operating decision maker, even if not included in that measure of segment profit or loss.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 27

-Subparagraph a

-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.

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- Definition

Amount of revenue from transactions with other operating segments of the same entity.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 27

-Subparagraph b

-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.

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