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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 3 Months Ended
Mar. 31, 2025
USD ($)
Goodwill [Line Items]  
Beginning Balance $ 12,270.2
Goodwill acquired during the period 258.0
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 53.8
Foreign currency translation adjustments during the period 132.7
Ending Balance 12,714.7
Brokerage [Member]  
Goodwill [Line Items]  
Beginning Balance 11,923.4
Goodwill acquired during the period 174.3
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 53.8
Foreign currency translation adjustments during the period 125.5
Ending Balance 12,277.0
Risk Management [Member]  
Goodwill [Line Items]  
Beginning Balance 328.3
Goodwill acquired during the period 83.7
Foreign currency translation adjustments during the period 7.2
Ending Balance 419.2
Corporate [Member]  
Goodwill [Line Items]  
Beginning Balance 18.5
Ending Balance $ 18.5