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Intangible assets - Summary of exploration and evaluation expenditure (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Exploration And Evaluation Expenditure [Abstract]      
Net expenditure in the year (net of cash proceeds of US$233 million (2017: US$3 million; 2016: US$208 million) on disposal of undeveloped projects) $ 345 $ 493 $ 284
Non-cash movements and non-cash proceeds on disposal of undeveloped projects 45 (24) (177)
Amount capitalised during the year 90 57 8
Net charge for the year (210) (460) (453)
Reconciliation to income statement      
Exploration and evaluation costs (488) (445) (497)
Profit/(loss) relating to interests in undeveloped projects 278 (15) 44
Net charge for the year $ (210) $ (460) $ (453)