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Intangible assets - Summary of exploration and evaluation expenditure (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of detailed information about intangible assets [Line Items] |  |  |
Net proceeds on disposal of undeveloped projects | $ 233 | $ 3 | $ 208
Valeria [Member] |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |
Gain on sales of undeveloped properties | 83 |  |
Winchester South [Member] |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |
Gain on sales of undeveloped properties | $ 195 |  |

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- Definition The cash inflow from the disposal of exploration and evaluation assets. [Refer: Exploration and evaluation assets [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 7 -IssueDate 2017-01-01 -Paragraph 16 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2017-03-01&anchor=para_16&doctype=Standard -URIDate 2017-03-09 + Details
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