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Intangible assets - Summary of exploration and evaluation expenditure (Parenthetical and Footnotes) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about intangible assets [Line Items]      
Proceeds from disposal of exploration and evaluation assets $ 1 $ 10 $ 233
Valeria      
Disclosure of detailed information about intangible assets [Line Items]      
Gain on sales of undeveloped properties     83
Winchester South      
Disclosure of detailed information about intangible assets [Line Items]      
Gain on sales of undeveloped properties     $ 195