Activity in Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Beginning balance, Goodwill gross	$ 3,765.4	$ 1,035.8
Additions/adjustments, gross		2,622.6
Foreign currency translation adjustments, gross	78.3	107.0
Ending balance, Goodwill gross	3,843.7	3,765.4
Beginning balance, Accumulated impairment charge	(12.2)	(12.2)
Ending balance, Accumulated impairment charge	(12.2)	(12.2)
Beginning balance, goodwill net	3,753.2	1,023.6
Additions/adjustments		2,622.6
Foreign currency translation adjustments	78.3	107.0
Ending balance, goodwill net	3,831.5	3,753.2
MIS [Member]
Goodwill [Line Items]
Beginning balance, Goodwill gross	285.2	277.0
Foreign currency translation adjustments, gross	6.4	8.2
Ending balance, Goodwill gross	291.6	285.2
Beginning balance, goodwill net	285.2	277.0
Foreign currency translation adjustments	6.4	8.2
Ending balance, goodwill net	291.6	285.2
MA [Member]
Goodwill [Line Items]
Beginning balance, Goodwill gross	3,480.2	758.8
Additions/adjustments, gross		2,622.6
Foreign currency translation adjustments, gross	71.9	98.8
Ending balance, Goodwill gross	3,552.1	3,480.2
Beginning balance, Accumulated impairment charge	(12.2)	(12.2)
Ending balance, Accumulated impairment charge	(12.2)	(12.2)
Beginning balance, goodwill net	3,468.0	746.6
Additions/adjustments		2,622.6
Foreign currency translation adjustments	71.9	98.8
Ending balance, goodwill net	$ 3,539.9	$ 3,468.0