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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 3 Months Ended
Mar. 31, 2024
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 11,475.6
Goodwill acquired during the period 187.3
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) (31.9)
Foreign currency translation adjustments during the period (98.9)
Ending Balance 11,532.1
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 11,217.8
Goodwill acquired during the period 187.3
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) (31.9)
Foreign currency translation adjustments during the period (92.2)
Ending Balance 11,281.0
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 238.8
Foreign currency translation adjustments during the period (6.7)
Ending Balance 232.1
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 19.0
Ending Balance $ 19.0