Share-based payments - Summary of Information about Awards Granted (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	Binomial valuation 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	Monte Carlo Simulation and Binomial valuation 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 2018-01-01
-Paragraph 47
-Subparagraph a
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2018-03-01&anchor=para_47_a_i&doctype=Standard
-URIDate 2018-03-16

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Period Type:	duration