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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance $ (2,499) $ (2,542)  
Interest (79) (79) $ (90)
Movement in the effect of the asset ceiling 52 (33) (1)
Currency exchange rate (loss)/gain 91 (135)  
Ending balance (1,551) (2,499) (2,542)
Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (1,501)    
Interest (44)    
Currency exchange rate (loss)/gain 57    
Ending balance (680) (1,501)  
Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (998)    
Interest (35)    
Currency exchange rate (loss)/gain 34    
Ending balance (871) (998)  
Effect of asset ceiling [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (111) (63)  
Interest (10) (6)  
Movement in the effect of the asset ceiling 52 (33)  
Arrangements divested and other transfers 57    
Currency exchange rate (loss)/gain 12 (9)  
Ending balance   (111) $ (63)
Effect of asset ceiling [member] | Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (111)    
Interest (10)    
Movement in the effect of the asset ceiling 52    
Arrangements divested and other transfers 57    
Currency exchange rate (loss)/gain $ 12    
Ending balance   $ (111)