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Retirement benefits - Additional Information (Detail)
£ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2025
USD ($)
Dec. 31, 2025
GBP (£)
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Actuarial assumption of expected rates of mortality | United Kingdom        
Disclosure of defined benefit plans [line items]        
Mortality - per annum long term trend 1.25%      
Pensioners male | United Kingdom        
Disclosure of defined benefit plans [line items]        
Actuarial assumption of mortality rates 0.91 0.91    
Pensioners female | United Kingdom        
Disclosure of defined benefit plans [line items]        
Actuarial assumption of mortality rates 0.85 0.85    
Non-pensioners male | United Kingdom        
Disclosure of defined benefit plans [line items]        
Actuarial assumption of mortality rates 0.86 0.86    
Non-pensioners female | United Kingdom        
Disclosure of defined benefit plans [line items]        
Actuarial assumption of mortality rates 0.84 0.84    
United Kingdom        
Disclosure of defined benefit plans [line items]        
Approximate extinguishment of the unfunded pension obligations 70.00%      
Bank accounts pledged as security $ 27 £ 20    
Philippines        
Disclosure of defined benefit plans [line items]        
Net retirement benefit asset 3   $ 3  
Plan assets 15   13  
Defined benefit obligation 12   10  
Unfunded Plans | Net defined benefit liability (asset) | Other post-employment benefit plans        
Disclosure of defined benefit plans [line items]        
Post-employment benefit plans $ 9   $ 7 $ 0