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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Goodwill [Line Items]    
Beginning Balance $ 11,475.6 $ 9,489.4
Goodwill acquired during the year 837.2 1,789.2
Goodwill adjustments related to appraisals and other acquisition adjustments 200.2 19.9
Goodwill written-off related to sales of business (5.8)  
Foreign currency translation adjustments during the year (237.0) 177.1
Ending Balance 12,270.2 11,475.6
Brokerage [Member]    
Goodwill [Line Items]    
Beginning Balance 11,217.8 9,358.1
Goodwill acquired during the year 829.7 1,667.4
Goodwill adjustments related to appraisals and other acquisition adjustments 98.7 20.0
Goodwill written-off related to sales of business (5.8)  
Foreign currency translation adjustments during the year (217.0) 172.3
Ending Balance 11,923.4 11,217.8
Risk Management [Member]    
Goodwill [Line Items]    
Beginning Balance 238.8 112.2
Goodwill acquired during the year 7.5 121.8
Goodwill adjustments related to appraisals and other acquisition adjustments 101.5 (0.1)
Foreign currency translation adjustments during the year (19.5) 4.9
Ending Balance 328.3 238.8
Corporate [Member]    
Goodwill [Line Items]    
Beginning Balance 19.0 19.1
Goodwill adjustments related to appraisals and other acquisition adjustments 0.0  
Foreign currency translation adjustments during the year (0.5) (0.1)
Ending Balance $ 18.5 $ 19.0