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Retirement benefits - Summary of Estimated Future Benefit Payments and Average Duration of Pension Obligations (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 103 $ 106
Within one year    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 5 5
Between one and five years    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 20 20
More than five years    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 78 $ 81
Plan | United Kingdom    
Disclosure of net defined benefit liability/asset [line items]    
Average duration of pension obligations (years) 12 years 12 years
Plan | United States    
Disclosure of net defined benefit liability/asset [line items]    
Average duration of pension obligations (years) 7 years 1 month 6 days 7 years 1 month 6 days
US post-retirement plan | United States    
Disclosure of net defined benefit liability/asset [line items]    
Average duration of pension obligations (years) 7 years 1 month 6 days 7 years 4 months 24 days