Revenue by Line of Business within Each Reportable Segment (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 816,100,000	$ 705,500,000	$ 2,456,800,000	$ 2,193,300,000
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Revenues	565,400,000	497,700,000	1,756,300,000	1,594,100,000
Intersegment revenue	22,300,000	19,600,000	65,700,000	57,500,000
Moodys Investors Service [Member] | Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenues	260,700,000	233,000,000	846,000,000	754,200,000
Moodys Investors Service [Member] | Structured Finance [Member]
Segment Reporting Information [Line Items]
Revenues	102,100,000	83,500,000	308,000,000	273,700,000
Moodys Investors Service [Member] | Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenues	91,800,000	78,900,000	269,400,000	249,900,000
Moodys Investors Service [Member] | Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenues	88,500,000	82,700,000	267,200,000	258,800,000
Moodys Investors Service [Member] | External Revenues
Segment Reporting Information [Line Items]
Revenues	543,100,000	478,100,000	1,690,600,000	1,536,600,000
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	276,400,000	230,400,000	776,200,000	665,200,000
Intersegment revenue	3,400,000	3,000,000	10,000,000	8,500,000
Moodys Analytics [Member] | Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	146,800,000	133,700,000	432,400,000	393,600,000
Moodys Analytics [Member] | Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	81,100,000	64,400,000	208,100,000	177,600,000
Moodys Analytics [Member] | Professional Services [Member]
Segment Reporting Information [Line Items]
Revenues	45,100,000	29,300,000	125,700,000	85,500,000
Moodys Analytics [Member] | External Revenues
Segment Reporting Information [Line Items]
Revenues	273,000,000	227,400,000	766,200,000	656,700,000
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	$ (25,700,000)	$ (22,600,000)	$ (75,700,000)	$ (66,000,000)