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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Mar. 31, 2024
Goodwill [Line Items]      
Total goodwill $ 12,714.7 $ 12,270.2 $ 11,532.1
Unites States [Member]      
Goodwill [Line Items]      
Total goodwill 7,143.9 7,040.4  
United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill 2,785.9 2,617.1  
Canada [Member]      
Goodwill [Line Items]      
Total goodwill 604.6 586.9  
Australia [Member]      
Goodwill [Line Items]      
Total goodwill 744.4 728.4  
New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill 202.5 191.7  
Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill 1,233.4 1,105.7  
Brokerage [Member]      
Goodwill [Line Items]      
Total goodwill 12,277.0 11,923.4  
Risk Management [Member]      
Goodwill [Line Items]      
Total goodwill 419.2 328.3  
Corporate [Member]      
Goodwill [Line Items]      
Total goodwill 18.5 18.5  
Operating Segments [Member] | Brokerage [Member]      
Goodwill [Line Items]      
Total goodwill 12,277.0 11,923.4 11,281.0
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill 7,069.1 6,965.6  
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill 2,671.6 2,591.4  
Operating Segments [Member] | Brokerage [Member] | Canada [Member]      
Goodwill [Line Items]      
Total goodwill 604.6 586.9  
Operating Segments [Member] | Brokerage [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill 523.0 509.1  
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill 193.8 183.2  
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill 1,214.9 1,087.2  
Operating Segments [Member] | Risk Management [Member]      
Goodwill [Line Items]      
Total goodwill 419.2 328.3 232.1
Operating Segments [Member] | Risk Management [Member] | Unites States [Member]      
Goodwill [Line Items]      
Total goodwill 74.8 74.8  
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member]      
Goodwill [Line Items]      
Total goodwill 114.3 25.7  
Operating Segments [Member] | Risk Management [Member] | Australia [Member]      
Goodwill [Line Items]      
Total goodwill 221.4 219.3  
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member]      
Goodwill [Line Items]      
Total goodwill 8.7 8.5  
Operating Segments [Member] | Corporate [Member]      
Goodwill [Line Items]      
Total goodwill 18.5 18.5 $ 19.0
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member]      
Goodwill [Line Items]      
Total goodwill $ 18.5 $ 18.5