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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 9 Months Ended
Sep. 30, 2023
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 9,489.4
Goodwill acquired during the period 957.9
Goodwill true-ups due to appraisals and other acquisition adjustments 9.8
Foreign currency translation adjustments during the period 12.3
Ending Balance 10,469.4
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 9,358.1
Goodwill acquired during the period 957.9
Goodwill true-ups due to appraisals and other acquisition adjustments 9.9
Foreign currency translation adjustments during the period 13.0
Ending Balance 10,338.9
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 112.2
Goodwill true-ups due to appraisals and other acquisition adjustments (0.1)
Foreign currency translation adjustments during the period (0.6)
Ending Balance 111.5
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 19.1
Foreign currency translation adjustments during the period (0.1)
Ending Balance $ 19.0