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Contingencies and commitments - Summary of aggregate amount of future payment commitments under purchase obligations outstanding (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of future payment commitments under purchase obligations [Line Items]    
Aggregate amount of future payment commitments under purchase obligations outstanding $ 19,789 $ 19,318
Due within 1 year [member]    
Disclosure of future payment commitments under purchase obligations [Line Items]    
Aggregate amount of future payment commitments under purchase obligations outstanding 2,818 2,677
Between 1 and 2 years [member]    
Disclosure of future payment commitments under purchase obligations [Line Items]    
Aggregate amount of future payment commitments under purchase obligations outstanding 1,664 1,711
Between 2 and 3 years [member]    
Disclosure of future payment commitments under purchase obligations [Line Items]    
Aggregate amount of future payment commitments under purchase obligations outstanding 1,433 1,327
Between 3 and 4 years [member]    
Disclosure of future payment commitments under purchase obligations [Line Items]    
Aggregate amount of future payment commitments under purchase obligations outstanding 1,318 1,207
Between 4 and 5 years [member]    
Disclosure of future payment commitments under purchase obligations [Line Items]    
Aggregate amount of future payment commitments under purchase obligations outstanding 1,062 1,040
More than 5 years [member]    
Disclosure of future payment commitments under purchase obligations [Line Items]    
Aggregate amount of future payment commitments under purchase obligations outstanding $ 11,494 $ 11,356