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Post-retirement benefits - Summary of total expense recognised in the income statement (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans $ (83) $ (79) $ (143)
Past service (cost)/credit (12) 87 0
Net interest on net defined benefit liability (32) (21) (36)
Non-investment expenses paid from the plans (20) (20) (13)
Total defined benefit expense (147) (33) (192)
Current employer service cost for defined contribution and industry-wide plans (458) (416) (367)
Total expense recognised in the income statement (605) $ (449) $ (559)
Pension benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (80)    
Past service (cost)/credit (9)    
Net interest on net defined benefit liability (2)    
Non-investment expenses paid from the plans (20)    
Total defined benefit expense (111)    
Current employer service cost for defined contribution and industry-wide plans (455)    
Total expense recognised in the income statement (566)    
Other benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (3)    
Past service (cost)/credit (3)    
Net interest on net defined benefit liability (30)    
Non-investment expenses paid from the plans 0    
Total defined benefit expense (36)    
Current employer service cost for defined contribution and industry-wide plans (3)    
Total expense recognised in the income statement $ (39)