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Financial instruments disclosures - Summary of fair value of financial instruments (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Disclosure Of Financial Instruments [Abstract]    
Short term borrowings, fair value $ (428) $ (552)
Medium and long term borrowings, fair value (13,806) (16,385)
Short term borrowings, carrying value (428) (552)
Medium and long term borrowings, carrying value $ (12,485) $ (14,624)