Activity in Goodwill (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Goodwill [Line Items]
Ending balance	$ 1,000.3	$ 665.2
Gross Goodwill [Member]
Goodwill [Line Items]
Beginning balance	677.4	649.3
Additions/adjustments	362.1	34.5
FX translation	(27.0)	(6.4)
Ending balance	1,012.5	677.4
Accumulated Impairment Charge [Member]
Goodwill [Line Items]
Beginning balance	(12.2)	(12.2)
Additions/adjustments	0	0
FX translation
Ending balance	(12.2)	(12.2)
Net Goodwill [Member]
Goodwill [Line Items]
Beginning balance	665.2	637.1
Additions/adjustments	362.1	34.5
FX translation	(27.0)	(6.4)
Ending balance	1,000.3	665.2
Moodys Investors Service [Member] | Gross Goodwill [Member]
Goodwill [Line Items]
Beginning balance	11.4	11.5
Additions/adjustments	291.1	0
FX translation	(7.7)	(0.1)
Ending balance	294.8	11.4
Moodys Investors Service [Member] | Accumulated Impairment Charge [Member]
Goodwill [Line Items]
Beginning balance
Additions/adjustments	0	0
FX translation
Ending balance	0
Moodys Investors Service [Member] | Net Goodwill [Member]
Goodwill [Line Items]
Beginning balance	11.4	11.5
Additions/adjustments	291.1	0
FX translation	(7.7)	(0.1)
Ending balance	294.8	11.4
Moodys Analytics [Member] | Gross Goodwill [Member]
Goodwill [Line Items]
Beginning balance	666.0	637.8
Additions/adjustments	71.0	34.5
FX translation	(19.3)	(6.3)
Ending balance	717.7	666.0
Moodys Analytics [Member] | Accumulated Impairment Charge [Member]
Goodwill [Line Items]
Beginning balance	(12.2)	(12.2)
Additions/adjustments	0	0
FX translation
Ending balance	(12.2)	(12.2)
Moodys Analytics [Member] | Net Goodwill [Member]
Goodwill [Line Items]
Beginning balance	653.8	625.6
Additions/adjustments	71.0	34.5
FX translation	(19.3)	(6.3)
Ending balance	$ 705.5	$ 653.8