Revenue by Line of Business within Each Reportable Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Revenue	$ 688.5	$ 531.3	$ 1,976.1	$ 1,713.6
MIS
Segment Reporting Information [Line Items]
Total external revenue	473.5	351.4	1,367.4	1,202.0
Intersegment royalty	17.8	16.8	52.4	49.0
Revenue	491.3	368.2	1,419.8	1,251.0
MIS \| Corporate finance (CFG)
Segment Reporting Information [Line Items]
Revenue	220.7	129.0	612.7	510.9
MIS \| Structured Finance (SFG)
Segment Reporting Information [Line Items]
Revenue	93.1	82.0	278.1	257.7
MIS \| Financial institutions (FIG)
Segment Reporting Information [Line Items]
Revenue	82.7	72.1	239.3	228.1
MIS \| Public, Project And Infrastructure Finance (PPIF)
Segment Reporting Information [Line Items]
Revenue	77.0	68.3	237.3	205.3
MA
Segment Reporting Information [Line Items]
Total external revenue	215.0	179.9	608.7	511.6
Intersegment royalty	3.1	2.6	9.0	7.8
Revenue	218.1	182.5	617.7	519.4
MA \| Research, Data And Analytics (RD&A)
Segment Reporting Information [Line Items]
Revenue	123.8	115.3	364.7	335.9
MA \| Enterprise Risk Solutions (ERS)
Segment Reporting Information [Line Items]
Revenue	64.0	50.8	163.6	135.7
MA \| Professional Services (PS)
Segment Reporting Information [Line Items]
Revenue	27.2	13.8	80.4	40.0
Eliminations
Segment Reporting Information [Line Items]
Revenue	$ (20.9)	$ (19.4)	$ (61.4)	$ (56.8)

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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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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