Financial Information by 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]
Instersegment Revenue	$ 816,100,000	$ 705,500,000	$ 2,456,800,000	$ 2,193,300,000
Operating, SG&A	443,200,000	390,600,000	1,293,800,000	1,200,500,000
Adjusted Operating Income	372,900,000	314,900,000	1,163,000,000	992,800,000
Depreciation and amortization	23,200,000	23,400,000	68,600,000	70,100,000
Operating Income	349,700,000	291,500,000	1,094,400,000	922,700,000
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Instersegment Revenue	565,400,000	497,700,000	1,756,300,000	1,594,100,000
Operating, SG&A	260,800,000	235,600,000	775,400,000	756,100,000
Adjusted Operating Income	304,600,000	262,100,000	980,900,000	838,000,000
Depreciation and amortization	11,500,000	12,100,000	34,300,000	34,900,000
Operating Income	293,100,000	250,000,000	946,600,000	803,100,000
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Instersegment Revenue	276,400,000	230,400,000	776,200,000	665,200,000
Operating, SG&A	208,100,000	177,600,000	594,100,000	510,400,000
Adjusted Operating Income	68,300,000	52,800,000	182,100,000	154,800,000
Depreciation and amortization	11,700,000	11,300,000	34,300,000	35,200,000
Operating Income	56,600,000	41,500,000	147,800,000
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Instersegment Revenue	(25,700,000)	(22,600,000)	(75,700,000)	(66,000,000)
Operating, SG&A	(25,700,000)	(22,600,000)	(75,700,000)	(66,000,000)
Adjusted Operating Income			0	0
Depreciation and amortization			0	0
Operating Income			$ 0	$ 0