Revenue by Line of Business within Each Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Revenues	$ 917.1	$ 834.9	$ 2,662.1	$ 2,618.6
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Revenues	637.6	571.6	1,836.9	1,859.1
Moodys Investors Service [Member] | Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenues	299.6	248.3	844.7	866.6
Moodys Investors Service [Member] | Structured Finance [Member]
Segment Reporting Information [Line Items]
Revenues	104.2	112.5	306.3	335.0
Moodys Investors Service [Member] | Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenues	95.8	89.5	280.4	273.7
Moodys Investors Service [Member] | Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenues	105.2	90.6	309.0	291.2
Moodys Investors Service [Member] | Rating Revenue [Member]
Segment Reporting Information [Line Items]
Revenues	604.8	540.9	1,740.4	1,766.5
Moodys Investors Service [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenues	7.5	7.2	22.6	23.1
Moodys Investors Service [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	612.3	548.1	1,763.0	1,789.6
Moodys Investors Service [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	25.3	23.5	73.9	69.5
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	309.0	290.1	908.9	838.7
Moodys Analytics [Member] | Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	167.7	157.9	500.9	465.0
Moodys Analytics [Member] | Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	101.5	92.2	288.5	252.5
Moodys Analytics [Member] | Professional Services [Member]
Segment Reporting Information [Line Items]
Revenues	35.6	36.7	109.7	111.5
Moodys Analytics [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	304.8	286.8	899.1	829.0
Moodys Analytics [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	4.2	3.3	9.8	9.7
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	$ (29.5)	$ (26.8)	$ (83.7)	$ (79.2)