Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 4,625.6
Goodwill acquired during the period	131.5
Goodwill adjustments due to appraisals and other acquisition adjustments	8.4
Goodwill written-off related to sales of business	(6.6)
Foreign currency translation adjustments during the period	42.0
Ending Balance	4,800.9
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	4,573.6
Goodwill acquired during the period	131.5
Goodwill adjustments due to appraisals and other acquisition adjustments	8.4
Goodwill written-off related to sales of business	(6.6)
Foreign currency translation adjustments during the period	41.5
Ending Balance	4,748.4
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	49.3
Foreign currency translation adjustments during the period	0.5
Ending Balance	49.8
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.7
Ending Balance	$ 2.7

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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