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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 9 Months Ended
Sep. 30, 2022
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 8,666.2
Goodwill acquired during the period 283.1
Goodwill true-ups due to appraisals and other acquisition adjustments 394.4
Foreign currency translation adjustments during the period (506.5)
Ending Balance 8,837.2
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 8,544.6
Goodwill acquired during the period 267.1
Goodwill true-ups due to appraisals and other acquisition adjustments 395.8
Foreign currency translation adjustments during the period (500.4)
Ending Balance 8,707.1
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 100.9
Goodwill acquired during the period 16.0
Goodwill true-ups due to appraisals and other acquisition adjustments (1.6)
Foreign currency translation adjustments during the period (5.0)
Ending Balance 110.3
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 20.7
Goodwill true-ups due to appraisals and other acquisition adjustments 0.2
Foreign currency translation adjustments during the period (1.1)
Ending Balance $ 19.8