Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Beginning Balance	$ 4,625.6	$ 4,164.8
Goodwill acquired during the year	975.7	584.6
Goodwill adjustments related to appraisals and other acquisition adjustments		(0.1)
Goodwill written-off related to sales of business	(7.2)
Foreign currency translation adjustments during the year	24.4	(123.7)
Ending Balance	5,618.5	4,625.6
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	4,573.6	4,119.2
Goodwill acquired during the year	958.4	574.7
Goodwill adjustments related to appraisals and other acquisition adjustments	0.2	2.2
Goodwill written-off related to sales of business	(7.2)
Foreign currency translation adjustments during the year	23.9	(122.5)
Ending Balance	5,548.9	4,573.6
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	49.3	42.6
Goodwill acquired during the year	16.9	9.9
Goodwill adjustments related to appraisals and other acquisition adjustments	(0.2)	(2.3)
Foreign currency translation adjustments during the year	0.6	(0.9)
Ending Balance	66.6	49.3
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.7	3.0
Goodwill acquired during the year	0.4
Foreign currency translation adjustments during the year	(0.1)	(0.3)
Ending Balance	$ 3.0	$ 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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