Summary Of Significant Accounting Policies [Line Items]

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Low end of range of time period over which revenue related to commercial mortgage-backed securities, derivatives, international residential mortgage-backed and asset-backed securities are deferred. For these securities, issuers can elect to pay monitoring fees upfront. These fees are deferred and recognized over the future monitoring periods based on the expected lives of the rated securities.

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The amount owed to the reporting entity by counterparties in securitized loan transactions.

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Description of risks that arise due to the volume of business transacted with a particular customer. At a minimum, the description informs financial statement users of the general nature of the risk, but excludes "Information about Major Customers" that may be disclosed elsewhere (for instance, segment disclosures).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
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Amount of revenue recognized that was previously reported as deferred or unearned revenue.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
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Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

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-Publisher FASB
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- Definition

Number of reporting units tested for impairment of goodwill. A reporting unit is an operating segment or one level below an operating segment.

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The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Research and development costs	$ 42.0	$ 40.1	$ 29.1
Deferred revenue related to rated securities	$ 140.1	133.0	121.0
Number of reporting units	7
Description of concentration risk, Customer	No customer accounted for 10% or more of accounts receivable at December 31, 2017 or 2016.
Retained Earnings [Member]
Summary Of Significant Accounting Policies [Line Items]
Effect of Adoption Adjustments to retained earnings	$ 4.6
Commercial Paper [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable related to accrued revenue	27.0	25.0	24.0
Monitoring Service [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable related to accrued revenue	$ 185.0	$ 159.1	$ 146.4
MIS [Member]
Summary Of Significant Accounting Policies [Line Items]
Weighted average expected lives of the rated securities	24 years
Number of reporting units	2
MA [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units	5