Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 1,233	$ 1,240	$ 1,214	$ 1,142	$ 1,060	$ 1,081	$ 1,175	$ 1,127	$ 4,829	$ 4,443	$ 4,204
Operating income	504	549	483	462	376	467	534	491	1,998	1,868	1,821
Net income attributable to Moody’s	$ 359	$ 380	$ 310	$ 373	$ 251	$ 310	$ 376	$ 373	$ 1,422	$ 1,310	$ 1,001
Basic (in USD per share)	$ 1.91	$ 2.01	$ 1.64	$ 1.96	$ 1.31	$ 1.62	$ 1.96	$ 1.95	$ 7.51	$ 6.84	$ 5.24
Diluted (in USD per share)	$ 1.88	$ 1.99	$ 1.62	$ 1.93	$ 1.29	$ 1.59	$ 1.94	$ 1.92	$ 7.42	$ 6.74	$ 5.15