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Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Mar. 31, 2025
Goodwill [Line Items]      
Total goodwill $ 22,958 $ 22,593 $ 12,715
United States      
Goodwill [Line Items]      
Total goodwill 15,777 16,537  
United Kingdom      
Goodwill [Line Items]      
Total goodwill 3,607 3,031  
Canada      
Goodwill [Line Items]      
Total goodwill 624 628  
Australia      
Goodwill [Line Items]      
Total goodwill 879 829  
New Zealand      
Goodwill [Line Items]      
Total goodwill 234 233  
Other foreign      
Goodwill [Line Items]      
Total goodwill 1,837 1,335  
Brokerage      
Goodwill [Line Items]      
Total goodwill 22,424 22,078  
Risk Management      
Goodwill [Line Items]      
Total goodwill 517 497  
Corporate      
Goodwill [Line Items]      
Total goodwill 17 18  
Operating Segments | Brokerage      
Goodwill [Line Items]      
Total goodwill 22,424 22,078 12,277
Operating Segments | Brokerage | United States      
Goodwill [Line Items]      
Total goodwill 15,668 16,428  
Operating Segments | Brokerage | United Kingdom      
Goodwill [Line Items]      
Total goodwill 3,468 2,889  
Operating Segments | Brokerage | Canada      
Goodwill [Line Items]      
Total goodwill 624 628  
Operating Segments | Brokerage | Australia      
Goodwill [Line Items]      
Total goodwill 632 591  
Operating Segments | Brokerage | New Zealand      
Goodwill [Line Items]      
Total goodwill 225 225  
Operating Segments | Brokerage | Other foreign      
Goodwill [Line Items]      
Total goodwill 1,807 1,317  
Operating Segments | Risk Management      
Goodwill [Line Items]      
Total goodwill 517 497 419
Operating Segments | Risk Management | United States      
Goodwill [Line Items]      
Total goodwill 109 109  
Operating Segments | Risk Management | United Kingdom      
Goodwill [Line Items]      
Total goodwill 139 142  
Operating Segments | Risk Management | Canada      
Goodwill [Line Items]      
Total goodwill 0 0  
Operating Segments | Risk Management | Australia      
Goodwill [Line Items]      
Total goodwill 247 238  
Operating Segments | Risk Management | New Zealand      
Goodwill [Line Items]      
Total goodwill 9 8  
Operating Segments | Risk Management | Other foreign      
Goodwill [Line Items]      
Total goodwill 13 0  
Operating Segments | Corporate      
Goodwill [Line Items]      
Total goodwill 17 18 $ 19
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 $ 17 $ 18