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Financial instruments and risk management - Summary of fair value of financial instruments (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Financial Instruments [Abstract]    
Short term borrowings and bank overdrafts $ (552) $ (717)
Medium and long-term borrowings (14,624) (16,913)
Short term borrowings, fair value (552) (706)
Medium and long term borrowings, fair value $ (16,385) $ (18,437)