Revenue by Line of Business within Each Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 918.1	$ 873.5	$ 1,783.7	$ 1,640.7
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	(26.8)	(25.2)	(52.4)	(50.0)
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Revenues	662.9	646.9	1,287.5	1,197.5
Moodys Investors Service [Member] | Corporate Finance [Member]
Segment Reporting Information [Line Items]
Revenues	319.6	320.9	618.3	585.3
Moodys Investors Service [Member] | Structured Finance [Member]
Segment Reporting Information [Line Items]
Revenues	121.2	110.6	222.5	205.9
Moodys Investors Service [Member] | Financial Institutions [Member]
Segment Reporting Information [Line Items]
Revenues	90.4	92.2	184.2	177.6
Moodys Investors Service [Member] | Public Project And Infrastructure Finance [Member]
Segment Reporting Information [Line Items]
Revenues	99.9	98.0	200.6	178.7
Moodys Investors Service [Member] | MIS Other [Member]
Segment Reporting Information [Line Items]
Revenues	8.1	3.3	15.9	6.6
Moodys Investors Service [Member] | External Revenues
Segment Reporting Information [Line Items]
Revenues	639.2	625.0	1,241.5	1,154.1
Moodys Investors Service [Member] | Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	23.7	21.9	46.0	43.4
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	282.0	251.8	548.6	493.2
Moodys Analytics [Member] | Research Data And Analytics [Member]
Segment Reporting Information [Line Items]
Revenues	157.5	141.4	307.1	279.0
Moodys Analytics [Member] | Enterprise Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	83.2	67.2	160.3	127.0
Moodys Analytics [Member] | Professional Services [Member]
Segment Reporting Information [Line Items]
Revenues	38.2	39.9	74.8	80.6
Moodys Analytics [Member] | External Revenues
Segment Reporting Information [Line Items]
Revenues	278.9	248.5	542.2	486.6
Moodys Analytics [Member] | Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues	$ 3.1	$ 3.3	$ 6.4	$ 6.6