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Intangible assets - Summary of exploration and evaluation expenditure (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Exploration And Evaluation Expenditure [Abstract]      
Net expenditure in the year (net of cash proceeds of US$10 million (2018: US$233 million; 2017: US$3 million) on disposal of undeveloped projects) $ (671) $ (345) $ (493)
Non-cash movements and non-cash proceeds on disposal of undeveloped projects 0 45 (24)
Amount capitalised during the year 57 90 57
Net charge for the year (614) (210) (460)
Reconciliation to income statement      
Exploration and evaluation costs (624) (488) (445)
Profit/(loss) relating to interests in undeveloped projects 10 278 (15)
Net charge for the year $ (614) $ (210) $ (460)