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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Goodwill [Line Items]
Beginning Balance	$ 6,127.0	$ 5,618.5
Goodwill acquired during the year	2,561.5	359.6
Goodwill adjustments related to appraisals and other acquisition adjustments	50.8	31.5
Goodwill written-off related to sales of business	(1.0)
Foreign currency translation adjustments during the year	(72.1)	117.4
Ending Balance	8,666.2	6,127.0
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	6,053.6	5,548.9
Goodwill acquired during the year	2,511.8	359.6
Goodwill adjustments related to appraisals and other acquisition adjustments	50.8	29.8
Goodwill written-off related to sales of business	(1.0)
Foreign currency translation adjustments during the year	(70.6)	115.3
Ending Balance	8,544.6	6,053.6
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	70.5	66.6
Goodwill acquired during the year	31.6
Goodwill adjustments related to appraisals and other acquisition adjustments		1.7
Foreign currency translation adjustments during the year	(1.2)	2.2
Ending Balance	100.9	70.5
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	2.9	3.0
Goodwill acquired during the year	18.1
Foreign currency translation adjustments during the year	(0.3)	(0.1)
Ending Balance	$ 20.7	$ 2.9

X

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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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