Revenue by Line of Business within Each Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Revenues	$ 928.9	$ 918.1	$ 1,745.0	$ 1,783.7
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Revenues	650.2	662.9	1,199.3	1,287.5
Moodys Investors Service [Member] | Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenues	304.8	319.6	545.1	618.3
Moodys Investors Service [Member] | Structured Finance [Member]
Segment Reporting Information [Line Items]
Revenues	111.5	121.2	202.1	222.5
Moodys Investors Service [Member] | Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenues	89.7	90.4	184.6	184.2
Moodys Investors Service [Member] | Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenues	112.3	99.9	203.8	200.6
Moodys Investors Service [Member] | Rating Revenue [Member]
Segment Reporting Information [Line Items]
Revenues	618.3	631.1	1,135.6	1,225.6
Moodys Investors Service [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenues	7.3	8.1	15.1	15.9
Moodys Investors Service [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	625.6	639.2	1,150.7	1,241.5
Moodys Investors Service [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	24.6	23.7	(48.6)	46.0
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	306.1	282.0	599.9	548.6
Moodys Analytics [Member] | Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	168.3	157.5	333.2	307.1
Moodys Analytics [Member] | Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	97.5	83.2	187.0	307.1
Moodys Analytics [Member] | Professional Services [Member]
Segment Reporting Information [Line Items]
Revenues	37.5	38.2	74.1	74.8
Moodys Analytics [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	303.3	278.9	594.3	542.2
Moodys Analytics [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	2.8	3.1	5.6	6.4
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	$ (27.4)	$ (26.8)	$ (54.2)	$ (52.4)