Principal associates - Additional information (Detail)	12 Months Ended
Dec. 31, 2017
Disclosure Of Significant Investments In Associates [Abstract]
Description of basis of preparation of summarised financial information of associate	summarised financial information is shown on a 100 per cent basis

The description of the basis of preparation of summarised financial information of an associate. [Refer: Associates [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2016-01-13
-Paragraph B15
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=12&date=2016-03-30&anchor=para_B15&doctype=Appendix&seq=2
-URIDate 2016-03-31

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration