Segment Information (MIS and MA Revenue By Line Of Business) (Details) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 531.3	$ 513.3	$ 1,713.6	$ 1,467.7
MIS [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	368.2	373.7	1,251.0	1,068.6
Intersegment royalty	16.8	15.5	49.0	46.3
External revenue	351.4	358.2	1,202.0	1,022.3
MIS [Member] \| Corporate Finance (CFG) [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
External revenue	129.0	144.9	510.9	399.2
MIS [Member] \| Structured Finance (SFG) [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
External revenue	82.0	70.1	257.7	214.7
MIS [Member] \| Financial Institutions (FIG) [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
External revenue	72.1	73.6	228.1	213.0
MIS [Member] \| Public, Project And Infrastructure Finance (PPIF) [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
External revenue	68.3	69.6	205.3	195.4
MA [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	182.5	157.4	519.4	452.4
Intersegment royalty	2.6	2.3	7.8	7.0
External revenue	179.9	155.1	511.6	445.4
MA [Member] \| Research, Data And Analytics (RD&A) [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
External revenue	115.3	106.0	335.9	315.8
MA [Member] \| Risk Management Software (RMS) [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
External revenue	47.9	42.8	127.5	115.3
MA [Member] \| Professional Services [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
External revenue	16.7	6.3	48.2	14.3
Eliminations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ (19.4)	$ (17.8)	$ (56.8)	$ (53.3)

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 03

-Paragraph 1

Reference 2: 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

+ Details

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Balance Type:	credit
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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.

+ Details

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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.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

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Namespace Prefix:	us-gaap
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