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Principal joint ventures - Summary of joint ventures (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Minera Escondida Limitada [member] |  |
Disclosure Of Joint Ventures [Line Items] |  |
Group interest (%), Joint Venture | 30.00% | 30.00%
Minera Escondida Limitada [member] | BHP Billiton Member |  |
Disclosure Of Joint Ventures [Line Items] |  |
Ownership interest in joint venture held by joint partner | 57.50% |
Sohar Aluminium Co. L.L.C. [member] |  |
Disclosure Of Joint Ventures [Line Items] |  |
Group interest (%), Joint Venture | 20.00% | 20.00%

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- Definition The proportion of ownership interest in a joint venture attributable to the entity. [Refer: Joint ventures [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 27 -IssueDate 2016-01-13 -Paragraph 17 -Subparagraph b -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2016-03-30&anchor=para_17_b_iii&doctype=Standard -URIDate 2016-03-31 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 27 -IssueDate 2016-01-13 -Paragraph 16 -Subparagraph b -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2016-03-30&anchor=para_16_b_iii&doctype=Standard -URIDate 2016-03-31 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 12 -IssueDate 2016-01-13 -Paragraph 21 -Subparagraph a -Clause iv -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2016-03-30&anchor=para_21_a_iv&doctype=Standard -URIDate 2016-03-31 + Details
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- Details
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