Quarterly Financial Data (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Data [Line Items]
Revenue	$ 754.2	$ 688.5	$ 640.8	$ 646.8	$ 567.1	$ 531.3	$ 605.2	$ 577.1	$ 2,730.3	$ 2,280.7	$ 2,032.0
Operating Income	260.2	269.7	278.5	269.0	172.1	196.1	270.1	250.1	1,077.4	888.4	772.8
Net income attributable to Moody's	$ 160.1	$ 183.9	$ 172.5	$ 173.5	$ 96.2	$ 130.7	$ 189.0	$ 155.5	$ 690.0	$ 571.4	$ 507.8
Basic	$ 0.72	$ 0.83	$ 0.77	$ 0.78	$ 0.43	$ 0.58	$ 0.83	$ 0.68	$ 3.09	$ 2.52	$ 2.16
Diluted	$ 0.70	$ 0.81	$ 0.76	$ 0.76	$ 0.43	$ 0.57	$ 0.82	$ 0.67	$ 3.05	$ 2.49	$ 2.15