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Share-based payments - Summary of Options Award Activity (Detail) - Stock options [member]
12 Months Ended
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Outstanding at January 1 19,894 20,940 20,940
Exercised (11,061) (1,046)
Forfeited (8,833)  
Outstanding at December 31   19,894 20,940