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Financial instruments and risk management - Summary of fair value of financial instruments (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Financial Instruments [Abstract]    
Short term borrowings, carrying value $ (312) $ (552)
Medium and long term borrowings, carrying value (12,440) (14,624)
Short term borrowings, fair value (312) (552)
Medium and long term borrowings, fair value $ (13,554) $ (16,385)