Revenue by Line of Business within Each Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 834.9	$ 816.1	$ 2,618.6	$ 2,456.8
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Revenues	571.6	568.8	1,859.1	1,766.3
Moodys Investors Service [Member] | Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenues	248.3	260.7	866.6	846.0
Moodys Investors Service [Member] | Structured Finance [Member]
Segment Reporting Information [Line Items]
Revenues	112.5	102.1	335.0	308.0
Moodys Investors Service [Member] | Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenues	89.5	91.8	273.7	269.4
Moodys Investors Service [Member] | Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenues	90.6	88.5	291.2	267.2
Moodys Investors Service [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenues	7.2	3.5	23.1	10.1
Moodys Investors Service [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	548.1	546.6	1,789.6	1,700.7
Moodys Investors Service [Member] | Rating Revenue [Member]
Segment Reporting Information [Line Items]
Revenues	540.9	543.1	1,766.5	1,690.6
Moodys Investors Service [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	23.5	22.2	69.5	65.6
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	290.1	273.0	838.7	766.2
Moodys Analytics [Member] | Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	157.9	143.3	465.0	422.3
Moodys Analytics [Member] | Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	92.2	81.1	252.5	208.1
Moodys Analytics [Member] | Professional Services [Member]
Segment Reporting Information [Line Items]
Revenues	36.7	45.1	111.5	125.7
Moodys Analytics [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	286.8	269.5	829.0	756.1
Moodys Analytics [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	3.3	3.5	9.7	10.1
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	$ (26.8)	$ (25.7)	$ (79.2)	$ (75.7)