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Post-retirement benefits - Summary of movement in effect of the asset ceiling (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance $ (1,551) $ (2,499)  
Interest (58) (79) $ (79)
Movement in the effect of the asset ceiling 0 52 (33)
Currency exchange rate (loss)/gain (20) 91  
Ending balance (1,730) (1,551) (2,499)
Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (680)    
Interest (22)    
Currency exchange rate (loss)/gain (3)    
Ending balance (831) (680)  
Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (871)    
Currency exchange rate (loss)/gain (17)    
Ending balance (899) (871)  
Effect of asset ceiling [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 0 (111)  
Interest 0 (10)  
Movement in the effect of the asset ceiling 0 52  
Arrangements divested and other transfers 0 57  
Currency exchange rate (loss)/gain 0 12  
Ending balance 0 0 $ (111)
Effect of asset ceiling [member] | Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 0    
Interest 0    
Movement in the effect of the asset ceiling 0    
Arrangements divested and other transfers 0    
Currency exchange rate (loss)/gain 0    
Ending balance $ 0 $ 0