Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Line Items]
Revenue	$ 865.9	$ 834.9	$ 918.1	$ 865.6	$ 3,484.5	$ 3,334.3	$ 2,972.5
Operating Income	333.1	349.7	419.3	371.3	1,473.4	1,439.1	1,234.6
Net income attributable to Moody's	$ 217.9	$ 231.6	$ 261.7	$ 230.1	$ 941.3	$ 988.7	$ 804.5
Basic	$ 1.11	$ 1.16	$ 1.3	$ 1.14	$ 4.7	$ 4.69	$ 3.67
Diluted	$ 1.09	$ 1.14	$ 1.28	$ 1.11	$ 4.63	$ 4.61	$ 3.6