Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary Of Significant Accounting Policies [Line Items]
Research and development costs	$ 37.9us-gaap_ResearchAndDevelopmentExpense	$ 22.8us-gaap_ResearchAndDevelopmentExpense	$ 16.1us-gaap_ResearchAndDevelopmentExpense
Deferred Revenue related to rated securities	107.0	97.0	82.0
Account receivable related to commercial paper	$ 22.0	$ 21.0	$ 22.0
Concentration risk, percentage	1000.00%us-gaap_ConcentrationRiskPercentage1	1000.00%us-gaap_ConcentrationRiskPercentage1
Reporting Units	6mco_ReportingUnits
MIS \| Ratings Member [Member]
Summary Of Significant Accounting Policies [Line Items]
Reporting Units	2
MIS \| Non Ratings [Member]
Summary Of Significant Accounting Policies [Line Items]
Reporting Units	1
MA
Summary Of Significant Accounting Policies [Line Items]
Reporting Units	4

- Definition

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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