Goodwill And Other Acquired Intangible Assets - Activity in Goodwill (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Beginning balance, Goodwill gross	$ 3,793.5	$ 3,765.4
Beginning balance, Accumulated impairment charge	(12.2)	(12.2)
Beginning balance, goodwill net	3,781.3	3,753.2
Additions/adjustments	97.0	211.5
Foreign currency translation adjustments	(104.4)	(183.4)
Ending balance, Goodwill gross	3,622.2	3,793.5
Ending balance, Accumulated impairment charge	(12.2)	(12.2)
Ending balance, goodwill net	3,610.0	3,781.3
Disposal Group, Held-for-sale, Not Discontinued Operations
Goodwill [Roll Forward]
Reclassification to assets held-for-sale	(163.9)
MIS
Goodwill [Roll Forward]
Beginning balance, Goodwill gross	257.8	285.2
Beginning balance, Accumulated impairment charge	0.0	0.0
Beginning balance, goodwill net	257.8	285.2
Additions/adjustments	52.1	0.0
Foreign currency translation adjustments	10.1	(27.4)
Ending balance, Goodwill gross	320.0	257.8
Ending balance, Accumulated impairment charge	0.0	0.0
Ending balance, goodwill net	320.0	257.8
MIS \| Disposal Group, Held-for-sale, Not Discontinued Operations
Goodwill [Roll Forward]
Reclassification to assets held-for-sale	0.0
MA
Goodwill [Roll Forward]
Beginning balance, Goodwill gross	3,535.7	3,480.2
Beginning balance, Accumulated impairment charge	(12.2)	(12.2)
Beginning balance, goodwill net	3,523.5	3,468.0
Additions/adjustments	44.9	211.5
Foreign currency translation adjustments	(114.5)	(156.0)
Ending balance, Goodwill gross	3,302.2	3,535.7
Ending balance, Accumulated impairment charge	(12.2)	(12.2)
Ending balance, goodwill net	3,290.0	$ 3,523.5
MA \| Disposal Group, Held-for-sale, Not Discontinued Operations
Goodwill [Roll Forward]
Reclassification to assets held-for-sale	$ (163.9)

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), classified as other, of an asset representing the future economic benefits 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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