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Post-retirement benefits - Summary of movement in present value of obligation (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Changes in net defined benefit liability (asset) [abstract]      
Beginning balance $ (470) $ (1,028)  
Changes in financial assumptions (losses)/gains 407 (3,410) $ (655)
Currency exchange rate gains/(losses) 10 (35)  
Ending balance (723) (470) (1,028)
Pension benefits      
Changes in net defined benefit liability (asset) [abstract]      
Beginning balance 152    
Currency exchange rate gains/(losses) 17    
Ending balance (95) 152  
Other benefits      
Changes in net defined benefit liability (asset) [abstract]      
Beginning balance (622)    
Currency exchange rate gains/(losses) (7)    
Ending balance (628) (622)  
Present value of defined benefit obligation      
Changes in net defined benefit liability (asset) [abstract]      
Beginning balance (11,177) (15,728)  
Current employer service costs (79) (143)  
Past service credit/(cost) 87 0  
Settlements 4 0  
Interest on obligation (533) (370)  
Contributions by plan participants (19) (20)  
Benefits paid 748 783  
Experience (losses)/gains (40) (170)  
Changes in financial assumptions (losses)/gains (418) 3,563  
Changes in demographic assumptions gains 51 17  
Currency exchange rate gains/(losses) (419) 891  
Ending balance (11,795) (11,177) $ (15,728)
Present value of defined benefit obligation | Pension benefits      
Changes in net defined benefit liability (asset) [abstract]      
Beginning balance (10,555)    
Current employer service costs (76)    
Past service credit/(cost) 88    
Settlements 4    
Interest on obligation (499)    
Contributions by plan participants (19)    
Benefits paid 716    
Experience (losses)/gains (69)    
Changes in financial assumptions (losses)/gains (393)    
Changes in demographic assumptions gains 48    
Currency exchange rate gains/(losses) (412)    
Ending balance (11,167) (10,555)  
Present value of defined benefit obligation | Other benefits      
Changes in net defined benefit liability (asset) [abstract]      
Beginning balance (622)    
Current employer service costs (3)    
Past service credit/(cost) (1)    
Settlements 0    
Interest on obligation (34)    
Contributions by plan participants 0    
Benefits paid 32    
Experience (losses)/gains 29    
Changes in financial assumptions (losses)/gains (25)    
Changes in demographic assumptions gains 3    
Currency exchange rate gains/(losses) (7)    
Ending balance $ (628) $ (622)