Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	9 Months Ended
Sep. 30, 2019 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 4,625.6
Goodwill acquired during the period	756.1
Goodwill adjustments due to appraisals and other acquisition adjustments	(1.8)
Goodwill written-off related to sales of business	(7.2)
Foreign currency translation adjustments during the period	(57.9)
Ending Balance	5,314.8
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	4,573.6
Goodwill acquired during the period	738.8
Goodwill adjustments due to appraisals and other acquisition adjustments	(1.3)
Goodwill written-off related to sales of business	(7.2)
Foreign currency translation adjustments during the period	(57.1)
Ending Balance	5,246.8
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	49.3
Goodwill acquired during the period	16.9
Goodwill adjustments due to appraisals and other acquisition adjustments	(0.5)
Foreign currency translation adjustments during the period	(0.8)
Ending Balance	64.9
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.7
Goodwill acquired during the period	0.4
Ending Balance	$ 3.1

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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