Revenue by Line of Business within Each Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 1,062.9	$ 917.1	$ 3,038.6	$ 2,662.1
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Revenues	723.2	637.6	2,131.1	1,836.9
Moodys Investors Service [Member] | Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenues	350.2	299.6	1,058.8	844.7
Moodys Investors Service [Member] | Structured Finance [Member]
Segment Reporting Information [Line Items]
Revenues	128.3	104.2	347.7	306.3
Moodys Investors Service [Member] | Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenues	102.1	95.8	316.8	280.4
Moodys Investors Service [Member] | Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenues	109.2	105.2	312.0	309.0
Moodys Investors Service [Member] | Rating Revenue [Member]
Segment Reporting Information [Line Items]
Revenues	689.8	604.8	2,035.3	1,740.4
Moodys Investors Service [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenues	4.4	7.5	13.8	22.6
Moodys Investors Service [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	694.2	612.3	2,049.1	1,763.0
Moodys Investors Service [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	29.0	25.3	82.0	73.9
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	372.8	309.0	1,001.1	908.9
Moodys Analytics [Member] | Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	218.4	167.7	574.7	500.9
Moodys Analytics [Member] | Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	112.6	101.5	305.8	288.5
Moodys Analytics [Member] | Professional Services [Member]
Segment Reporting Information [Line Items]
Revenues	37.7	35.6	109.0	109.7
Moodys Analytics [Member] | External Revenues [Member]
Segment Reporting Information [Line Items]
Revenues	368.7	304.8	989.5	899.1
Moodys Analytics [Member] | Intersegment Royality [Member]
Segment Reporting Information [Line Items]
Revenues	4.1	4.2	11.6	9.8
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	$ (33.1)	$ (29.5)	$ (93.6)	$ (83.7)