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Intangible assets - Additional information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about intangible assets [Line Items]      
Exploration and evaluation costs $ 445 $ 497 $ 576
Exploration and evaluation assets [Member]      
Disclosure of detailed information about intangible assets [Line Items]      
Capitalised related to projects which had not yet been approved to proceed 420 726  
Exploration and evaluation costs 393 711  
Early works expenditure within property, plant and equipment $ 27 $ 15