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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Jun. 30, 2024
Goodwill [Line Items]      
Total goodwill $ 13,740.3 $ 12,270.2 $ 11,915.5
United States      
Goodwill [Line Items]      
Total goodwill 7,812.0 7,040.4  
United Kingdom      
Goodwill [Line Items]      
Total goodwill 2,946.8 2,617.1  
Canada      
Goodwill [Line Items]      
Total goodwill 630.8 586.9  
Australia      
Goodwill [Line Items]      
Total goodwill 795.5 728.4  
New Zealand      
Goodwill [Line Items]      
Total goodwill 236.6 191.7  
Other foreign      
Goodwill [Line Items]      
Total goodwill 1,318.6 1,105.7  
Brokerage      
Goodwill [Line Items]      
Total goodwill 13,282.9 11,923.4  
Risk Management      
Goodwill [Line Items]      
Total goodwill 439.0 328.3  
Corporate      
Goodwill [Line Items]      
Total goodwill 18.4 18.5  
Operating Segments | Brokerage      
Goodwill [Line Items]      
Total goodwill 13,282.9 11,923.4 11,559.7
Operating Segments | Brokerage | United States      
Goodwill [Line Items]      
Total goodwill 7,737.2 6,965.6  
Operating Segments | Brokerage | United Kingdom      
Goodwill [Line Items]      
Total goodwill 2,820.3 2,591.4  
Operating Segments | Brokerage | Canada      
Goodwill [Line Items]      
Total goodwill 630.8 586.9  
Operating Segments | Brokerage | Australia      
Goodwill [Line Items]      
Total goodwill 566.9 509.1  
Operating Segments | Brokerage | New Zealand      
Goodwill [Line Items]      
Total goodwill 227.5 183.2  
Operating Segments | Brokerage | Other foreign      
Goodwill [Line Items]      
Total goodwill 1,300.2 1,087.2  
Operating Segments | Risk Management      
Goodwill [Line Items]      
Total goodwill 439.0 328.3 336.9
Operating Segments | Risk Management | United States      
Goodwill [Line Items]      
Total goodwill 74.8 74.8  
Operating Segments | Risk Management | United Kingdom      
Goodwill [Line Items]      
Total goodwill 126.5 25.7  
Operating Segments | Risk Management | Canada      
Goodwill [Line Items]      
Total goodwill 0.0 0.0  
Operating Segments | Risk Management | Australia      
Goodwill [Line Items]      
Total goodwill 228.6 219.3  
Operating Segments | Risk Management | New Zealand      
Goodwill [Line Items]      
Total goodwill 9.1 8.5  
Operating Segments | Risk Management | Other foreign      
Goodwill [Line Items]      
Total goodwill 0.0 0.0  
Operating Segments | Corporate      
Goodwill [Line Items]      
Total goodwill 18.4 18.5 $ 18.9
Operating Segments | Corporate | United States      
Goodwill [Line Items]      
Total goodwill 0.0 0.0  
Operating Segments | Corporate | United Kingdom      
Goodwill [Line Items]      
Total goodwill 0.0 0.0  
Operating Segments | Corporate | Canada      
Goodwill [Line Items]      
Total goodwill 0.0 0.0  
Operating Segments | Corporate | Australia      
Goodwill [Line Items]      
Total goodwill 0.0 0.0  
Operating Segments | Corporate | New Zealand      
Goodwill [Line Items]      
Total goodwill 0.0 0.0  
Operating Segments | Corporate | Other foreign      
Goodwill [Line Items]      
Total goodwill $ 18.4 $ 18.5