v3.26.3
Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Sep. 30, 2025
Goodwill [Line Items]      
Total goodwill $ 23,385 $ 22,593 $ 22,213
United States      
Goodwill [Line Items]      
Total goodwill 15,926 16,537  
United Kingdom      
Goodwill [Line Items]      
Total goodwill 3,662 3,031  
Canada      
Goodwill [Line Items]      
Total goodwill 883 628  
Australia      
Goodwill [Line Items]      
Total goodwill 895 829  
New Zealand      
Goodwill [Line Items]      
Total goodwill 231 233  
Other foreign      
Goodwill [Line Items]      
Total goodwill 1,788 1,335  
Brokerage      
Goodwill [Line Items]      
Total goodwill 22,839 22,078  
Risk Management      
Goodwill [Line Items]      
Total goodwill 530 497  
Corporate      
Goodwill [Line Items]      
Total goodwill 16 18  
Operating Segments | Brokerage      
Goodwill [Line Items]      
Total goodwill 22,839 22,078 21,739
Operating Segments | Brokerage | United States      
Goodwill [Line Items]      
Total goodwill 15,817 16,428  
Operating Segments | Brokerage | United Kingdom      
Goodwill [Line Items]      
Total goodwill 3,514 2,889  
Operating Segments | Brokerage | Canada      
Goodwill [Line Items]      
Total goodwill 883 628  
Operating Segments | Brokerage | Australia      
Goodwill [Line Items]      
Total goodwill 643 591  
Operating Segments | Brokerage | New Zealand      
Goodwill [Line Items]      
Total goodwill 223 225  
Operating Segments | Brokerage | Other foreign      
Goodwill [Line Items]      
Total goodwill 1,759 1,317  
Operating Segments | Risk Management      
Goodwill [Line Items]      
Total goodwill 530 497 456
Operating Segments | Risk Management | United States      
Goodwill [Line Items]      
Total goodwill 109 109  
Operating Segments | Risk Management | United Kingdom      
Goodwill [Line Items]      
Total goodwill 148 142  
Operating Segments | Risk Management | Canada      
Goodwill [Line Items]      
Total goodwill 0 0  
Operating Segments | Risk Management | Australia      
Goodwill [Line Items]      
Total goodwill 252 238  
Operating Segments | Risk Management | New Zealand      
Goodwill [Line Items]      
Total goodwill 8 8  
Operating Segments | Risk Management | Other foreign      
Goodwill [Line Items]      
Total goodwill 13 0  
Operating Segments | Corporate      
Goodwill [Line Items]      
Total goodwill 16 18 $ 18
Operating Segments | Corporate | United States      
Goodwill [Line Items]      
Total goodwill 0 0  
Operating Segments | Corporate | United Kingdom      
Goodwill [Line Items]      
Total goodwill 0 0  
Operating Segments | Corporate | Canada      
Goodwill [Line Items]      
Total goodwill 0 0  
Operating Segments | Corporate | Australia      
Goodwill [Line Items]      
Total goodwill 0 0  
Operating Segments | Corporate | New Zealand      
Goodwill [Line Items]      
Total goodwill 0 0  
Operating Segments | Corporate | Other foreign      
Goodwill [Line Items]      
Total goodwill $ 16 $ 18