Capital and other commitments - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Capital commitments [abstract]
Estimated outstanding commitment to invest in associates	$ 33,000,000	$ 36,000,000
Unutilised facilities provided by banks	$ 5,000,000	$ 0

The amount of outstanding commitments made by the entity in related party transactions. [Refer: Related parties [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2017-01-01
-Paragraph 18
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2017-03-01&anchor=para_18_b&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_OutstandingCommitmentsMadeByEntityRelatedPartyTransactions
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant