Goodwill And Other Acquired Intangible Assets - Activity in Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Activity In Goodwill [Abstract]
Beginning balance, goodwill gross	$ 3,765.4	$ 1,035.8
Ending balance, goodwill gross	3,793.5	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	211.5	2,622.6
Foreign currency translation adjustments	(183.4)	107.0
Ending balance, goodwill net	3,781.3	3,753.2
MIS [Member]
Activity In Goodwill [Abstract]
Beginning balance, goodwill gross	285.2	277.0
Ending balance, goodwill gross	257.8	285.2
Beginning balance, goodwill net	285.2	277.0
Foreign currency translation adjustments	(27.4)	8.2
Ending balance, goodwill net	257.8	285.2
MA [Member]
Activity In Goodwill [Abstract]
Beginning balance, goodwill gross	3,480.2	758.8
Ending balance, goodwill gross	3,535.7	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	211.5	2,622.6
Foreign currency translation adjustments	(156.0)	98.8
Ending balance, goodwill net	$ 3,523.5	$ 3,468.0

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount before accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of increase (decrease) from foreign currency translation adjustments and purchase accounting adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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