Activity in Goodwill (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Goodwill [Line Items]
Beginning balance, Goodwill gross	$ 1,033.3	$ 677.4
Additions/adjustments, gross	8.2	397.8
Foreign currency translation adjustments, gross	(39.3)	(41.9)
Ending balance, Goodwill gross	1,002.2	1,033.3
Beginning balance, Impairment Loss	(12.2)	(12.2)
Ending balance, Impairment Loss	(12.2)	(12.2)
Beginning balance	1,021.1	665.2
Additions/adjustments	8.2	397.8
Foreign currency translation adjustments	(39.3)	(41.9)
Ending balance	990.0	1,021.1
Moodys Investors Service [Member]
Goodwill [Line Items]
Beginning balance, Goodwill gross	298.7	11.4
Additions/adjustments, gross	4.9	296.7
Foreign currency translation adjustments, gross	(10.7)	(9.4)
Ending balance, Goodwill gross	$ 292.9	$ 298.7
Beginning balance, Impairment Loss
Ending balance, Impairment Loss
Beginning balance	$ 298.7	$ 11.4
Additions/adjustments	4.9	296.7
Foreign currency translation adjustments	(10.7)	(9.4)
Ending balance	292.9	298.7
Moodys Analytics [Member]
Goodwill [Line Items]
Beginning balance, Goodwill gross	298.7	666.0
Additions/adjustments, gross	3.3	101.1
Foreign currency translation adjustments, gross	(28.6)	(32.5)
Ending balance, Goodwill gross	709.3	298.7
Beginning balance, Impairment Loss	(12.2)	(12.2)
Ending balance, Impairment Loss	(12.2)	(12.2)
Beginning balance	722.4	653.8
Additions/adjustments	3.3	101.1
Foreign currency translation adjustments	(28.6)	(32.5)
Ending balance	$ 697.1	$ 722.4