Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Beginning Balance	$ 5,618.5	$ 4,625.6
Goodwill acquired during the year	359.6	975.7
Goodwill adjustments related to appraisals and other acquisition adjustments	31.5
Goodwill written-off related to sales of business		(7.2)
Foreign currency translation adjustments during the year	117.4	24.4
Ending Balance	6,127.0	5,618.5
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	5,548.9	4,573.6
Goodwill acquired during the year	359.6	958.4
Goodwill adjustments related to appraisals and other acquisition adjustments	29.8	0.2
Goodwill written-off related to sales of business		(7.2)
Foreign currency translation adjustments during the year	115.3	23.9
Ending Balance	6,053.6	5,548.9
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	66.6	49.3
Goodwill acquired during the year		16.9
Goodwill adjustments related to appraisals and other acquisition adjustments	1.7	(0.2)
Foreign currency translation adjustments during the year	2.2	0.6
Ending Balance	70.5	66.6
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	3.0	2.7
Goodwill acquired during the year		0.4
Foreign currency translation adjustments during the year	(0.1)	(0.1)
Ending Balance	$ 2.9	$ 3.0

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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