Revenue by Line of Business within Each Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 1,000.5	$ 928.9	$ 1,975.7	$ 1,745.0
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Revenues	713.7	650.3	1,407.9	1,199.4
Moodys Investors Service [Member] | Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenues	355.8	304.9	708.6	545.2
Moodys Investors Service [Member] | Structured Finance [Member]
Segment Reporting Information [Line Items]
Revenues	119.2	111.5	219.4	202.1
Moodys Investors Service [Member] | Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenues	102.4	89.7	214.7	184.6
Moodys Investors Service [Member] | Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenues	104.7	112.3	202.8	203.8
Moodys Investors Service [Member] | Rating Revenue [Member]
Segment Reporting Information [Line Items]
Revenues	682.1	618.4	1,345.5	1,135.7
Moodys Investors Service [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenues	4.6	7.3	9.4	15.1
Moodys Investors Service [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	686.7	625.7	1,354.9	1,150.8
Moodys Investors Service [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	27.0	24.6	53.0	48.6
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	317.6	306.0	628.3	599.8
Moodys Analytics [Member] | Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	180.9	168.3	356.3	333.2
Moodys Analytics [Member] | Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	97.3	97.5	193.2	187.0
Moodys Analytics [Member] | Professional Services [Member]
Segment Reporting Information [Line Items]
Revenues	35.6	37.4	71.3	74.0
Moodys Analytics [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	313.8	303.2	620.8	594.2
Moodys Analytics [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	3.8	2.8	7.5	5.6
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	$ (30.8)	$ (27.4)	$ (60.5)	$ (54.2)