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Retirement benefits - Defined Benefit Plan (Detail) - Net defined benefit liability (asset) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Changes in net defined benefit liability (asset) [abstract]      
At 1 January $ 68 $ 66 $ 66
Group contributions (6) (6) (5)
Interest expense recognised in profit or loss 6 5 3
Actuarial (gains)/losses recognised in other comprehensive income 0 (4) 2
Exchange and other adjustments 1 7 0
At 31 December 69 68 66
Net defined benefit liability (asset) 69 68 66
United Kingdom | Unfunded Plans      
Changes in net defined benefit liability (asset) [abstract]      
Benefit obligation, unfunded plans 18 17 19
United States | Unfunded Plans      
Changes in net defined benefit liability (asset) [abstract]      
Benefit obligation, unfunded plans 30 31 34
Post-employment benefit plans 12 13 13
Other post-employment benefit plans | Unfunded Plans      
Changes in net defined benefit liability (asset) [abstract]      
Post-employment benefit plans $ 9 $ 7 $ 0