Principal joint ventures - Summary of joint ventures (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
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%

The proportion of ownership interest in a joint venture attributable to the entity. [Refer: Joint ventures [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 27
-IssueDate 2017-01-01
-Paragraph 17
-Subparagraph b
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2017-03-01&anchor=para_17_b_iii&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 27
-IssueDate 2017-01-01
-Paragraph 16
-Subparagraph b
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=27&date=2017-03-01&anchor=para_16_b_iii&doctype=Standard
-URIDate 2017-03-09

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2017-01-01
-Paragraph 21
-Subparagraph a
-Clause iv
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2017-03-01&anchor=para_21_a_iv&doctype=Standard
-URIDate 2017-03-09

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Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration