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Intangible assets - Summary of exploration and evaluation expenditure (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Exploration And Evaluation Expenditure [Abstract]      
Net expenditure in the year (net of cash proceeds of US$1 million (2019: US$10 million; 2018: US$233 million) on disposal of undeveloped projects) $ (711) $ (671) $ (345)
Non-cash movements and non-cash proceeds on disposal of undeveloped projects 0 0 45
Exploration and evaluation costs capitalised 87 57 90
Net charge for the year (624) (614) (210)
Reconciliation to income statement:      
Exploration and evaluation costs (625) (624) (488)
Profit/(loss) relating to interests in undeveloped projects 1 10 278
Net charge for the year $ (624) $ (614) $ (210)