XML 58 R47.htm IDEA: XBRL DOCUMENT v3.25.3
Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Sep. 30, 2024
Goodwill [Line Items]      
Total goodwill $ 22,213.3 $ 12,270.2 $ 12,193.4
United States      
Goodwill [Line Items]      
Total goodwill 16,306.1 7,040.4  
United Kingdom      
Goodwill [Line Items]      
Total goodwill 2,921.0 2,617.1  
Canada      
Goodwill [Line Items]      
Total goodwill 619.5 586.9  
Australia      
Goodwill [Line Items]      
Total goodwill 809.2 728.4  
New Zealand      
Goodwill [Line Items]      
Total goodwill 230.3 191.7  
Other foreign      
Goodwill [Line Items]      
Total goodwill 1,327.2 1,105.7  
Brokerage      
Goodwill [Line Items]      
Total goodwill 21,738.9 11,923.4  
Risk Management      
Goodwill [Line Items]      
Total goodwill 456.6 328.3  
Corporate      
Goodwill [Line Items]      
Total goodwill 17.8 18.5  
Operating Segments | Brokerage      
Goodwill [Line Items]      
Total goodwill 21,738.9 11,923.4 11,829.3
Operating Segments | Brokerage | United States      
Goodwill [Line Items]      
Total goodwill 16,231.3 6,965.6  
Operating Segments | Brokerage | United Kingdom      
Goodwill [Line Items]      
Total goodwill 2,780.0 2,591.4  
Operating Segments | Brokerage | Canada      
Goodwill [Line Items]      
Total goodwill 619.5 586.9  
Operating Segments | Brokerage | Australia      
Goodwill [Line Items]      
Total goodwill 577.2 509.1  
Operating Segments | Brokerage | New Zealand      
Goodwill [Line Items]      
Total goodwill 221.5 183.2  
Operating Segments | Brokerage | Other foreign      
Goodwill [Line Items]      
Total goodwill 1,309.4 1,087.2  
Operating Segments | Risk Management      
Goodwill [Line Items]      
Total goodwill 456.6 328.3 345.2
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 141.0 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 232.0 219.3  
Operating Segments | Risk Management | New Zealand      
Goodwill [Line Items]      
Total goodwill 8.8 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 17.8 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 $ 17.8 $ 18.5