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Share-based payments - Summary of Movements in Number of Share Options Outstanding and Weighted Average Exercise Prices (Detail) - Employee stock option plan [member]
12 Months Ended
Dec. 31, 2018
CAD ($)
shares
Dec. 31, 2017
CAD ($)
shares
Disclosure Of Number And Weighted Average Exercise Price Of Outstanding Share Options [Line Items]    
Weighted average exercise price, At January 1, | $ $ 30.18 $ 37.75
Weighted average exercise price, Regular options granted | $ 6.20 22.15
Weighted average exercise price, Exercised | $ 0.00 16.10
Weighted average exercise price, Expired | $ 51.46 73.89
Weighted average exercise price, Forfeited | $ 26.99 33.78
Weighted average exercise price, At December 31, | $ $ 22.56 $ 30.18
Number of options, At January 1, | shares 5,944,510 5,779,205
Number of options, Regular options granted | shares 1,078,797 1,160,905
Number of options, Exercised | shares 0 (48,529)
Number of options, Expired | shares (870,904) (788,672)
Number of options, Forfeited | shares (561,175) (158,399)
Number of options, At December 31, | shares 5,591,228 5,944,510