Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Line Items]
Beginning Balance	$ 9,489.4	$ 8,666.2
Goodwill acquired during the year	1,789.2	709.9
Goodwill adjustments related to appraisals and other acquisition adjustments	19.9	426.9
Foreign currency translation adjustments during the year	177.1	(313.6)
Ending Balance	11,475.6	9,489.4
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	9,358.1	8,544.6
Goodwill acquired during the year	1,667.4	693.9
Goodwill adjustments related to appraisals and other acquisition adjustments	20.0	428.3
Foreign currency translation adjustments during the year	172.3	(308.7)
Ending Balance	11,217.8	9,358.1
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	112.2	100.9
Goodwill acquired during the year	121.8	16.0
Goodwill adjustments related to appraisals and other acquisition adjustments	(0.1)	(1.6)
Foreign currency translation adjustments during the year	4.9	(3.1)
Ending Balance	238.8	112.2
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	19.1	20.7
Goodwill adjustments related to appraisals and other acquisition adjustments		0.2
Foreign currency translation adjustments during the year	(0.1)	(1.8)
Ending Balance	$ 19.0	$ 19.1