Investment in associate - Additional information (Details) - t t in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Atlas Methanol Company Unlimited
Disclosure of associates [line items]
Annual methanol production facility owned (in tonnes)	1.8
Atlas Methanol Company Unlimited \| Fixed-price contracts \| Methanol \| 2005 through 2014
Disclosure of associates [line items]
Percentage of sales	50.00%
Atlas Methanol Company Unlimited \| Fixed-price contracts \| Methanol \| Late 2014 through 2019
Disclosure of associates [line items]
Percentage of sales	10.00%
Atlas Methanol Company Unlimited
Disclosure of associates [line items]
Percentage of ownership interest in associates	63.10%	63.10%

The proportion of ownership interest in an associate attributable to the entity. [Refer: Associates [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