Share-based payments - Summary of Information about Awards Granted (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2017
Annual Performance Plan [member]
Information regarding option pricing models and assumptions used for grants during the year [line items]
Description of option pricing model	Binomial valuation model
Long Term Incentive Plan [member]
Information regarding option pricing models and assumptions used for grants during the year [line items]
Description of option pricing model	Monte Carlo Simulation and Binomial valuation model

The description of the option pricing model used for share options granted. [Refer: Option pricing model [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2017-01-01
-Paragraph 47
-Subparagraph a
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_47_a_i&doctype=Standard
-URIDate 2017-03-09

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