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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 6 Months Ended
Jun. 30, 2025
USD ($)
Goodwill [Roll Forward]  
Beginning Balance $ 12,270.2
Goodwill acquired during the period 1,027.7
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 40.2
Foreign currency translation adjustments during the period 402.2
Ending Balance 13,740.3
Brokerage  
Goodwill [Roll Forward]  
Beginning Balance 11,923.4
Goodwill acquired during the period 944.0
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 34.3
Foreign currency translation adjustments during the period 381.2
Ending Balance 13,282.9
Risk Management  
Goodwill [Roll Forward]  
Beginning Balance 328.3
Goodwill acquired during the period 83.7
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 5.9
Foreign currency translation adjustments during the period 21.1
Ending Balance 439.0
Corporate  
Goodwill [Roll Forward]  
Beginning Balance 18.5
Goodwill acquired during the period 0.0
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 0.0
Foreign currency translation adjustments during the period (0.1)
Ending Balance $ 18.4