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Intangible Assets - Changes in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2024
Dec. 31, 2023
Goodwill [Line Items]    
Beginning Balance $ 11,475.6  
Goodwill acquired during the period 341.0  
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 196.4 $ 256.7
Goodwill written-off related to sales of business and impairment (5.8)  
Foreign currency translation adjustments during the period (91.7)  
Ending Balance 11,915.5 11,475.6
Brokerage [Member]    
Goodwill [Line Items]    
Beginning Balance 11,217.8  
Goodwill acquired during the period 341.0  
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 94.4  
Goodwill written-off related to sales of business and impairment (5.8)  
Foreign currency translation adjustments during the period (87.7)  
Ending Balance 11,559.7 11,217.8
Risk Management [Member]    
Goodwill [Line Items]    
Beginning Balance 238.8  
Goodwill true-ups due to appraisals and other acquisition adjustments (see Note 3) 102.0  
Foreign currency translation adjustments during the period (3.9)  
Ending Balance 336.9 238.8
Corporate [Member]    
Goodwill [Line Items]    
Beginning Balance 19.0  
Foreign currency translation adjustments during the period (0.1)  
Ending Balance $ 18.9 $ 19.0