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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($)
Goodwill [Line Items]
Beginning Balance	$ 9,489.4
Goodwill acquired during the period	181.2
Goodwill true-ups due to appraisals and other acquisition adjustments	7.5
Foreign currency translation adjustments during the period	25.1
Ending Balance	9,703.2
Brokerage [Member]
Goodwill [Line Items]
Beginning Balance	9,358.1
Goodwill acquired during the period	181.2
Goodwill true-ups due to appraisals and other acquisition adjustments	7.5
Foreign currency translation adjustments during the period	25.0
Ending Balance	9,571.8
Risk Management [Member]
Goodwill [Line Items]
Beginning Balance	112.2
Ending Balance	112.2
Corporate [Member]
Goodwill [Line Items]
Beginning Balance	19.1
Foreign currency translation adjustments during the period	0.1
Ending Balance	$ 19.2

X

- Definition

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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