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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance $ (2,542) $ (2,530)  
Interest (79) (90) $ (113)
Movement in the effect of the asset ceiling 33 1 8
Currency exchange rate (loss)/gain (135) (113)  
Ending balance (2,499) (2,542) (2,530)
Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (1,616)    
Interest (45)    
Currency exchange rate (loss)/gain (104)    
Ending balance (1,501) (1,616)  
Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (926)    
Interest (34)    
Currency exchange rate (loss)/gain (31)    
Ending balance (998) (926)  
Effect of asset ceiling [Member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (63) (49)  
Interest (6) (5)  
Movement in the effect of the asset ceiling (33) (1)  
Currency exchange rate (loss)/gain (9) (8)  
Ending balance (111) (63) $ (49)
Effect of asset ceiling [Member] | Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (63)    
Interest (6)    
Movement in the effect of the asset ceiling (33)    
Currency exchange rate (loss)/gain (9)    
Ending balance $ (111) $ (63)