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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail)
$ in Millions
 9 Months Ended
Sep. 30, 2025
USD ($)
Goodwill [Roll Forward]  
Beginning Balance $ 12,270.2
Goodwill acquired during the period 9,539.9
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 40.4
Goodwill written-off related to sales of business and impairment (3.0)
Foreign currency translation adjustments during the period 365.8
Ending Balance 22,213.3
Brokerage  
Goodwill [Roll Forward]  
Beginning Balance 11,923.4
Goodwill acquired during the period 9,440.3
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 33.7
Goodwill written-off related to sales of business and impairment (3.0)
Foreign currency translation adjustments during the period 344.5
Ending Balance 21,738.9
Risk Management  
Goodwill [Roll Forward]  
Beginning Balance 328.3
Goodwill acquired during the period 99.6
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 6.7
Goodwill written-off related to sales of business and impairment 0.0
Foreign currency translation adjustments during the period 22.0
Ending Balance 456.6
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
Goodwill written-off related to sales of business and impairment 0.0
Foreign currency translation adjustments during the period (0.7)
Ending Balance $ 17.8