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Shareholders' capital - Stock Option Activity (Detail)
CAD / shares in Units, CAD in Thousands
 12 Months Ended
Dec. 31, 2017
CAD
CAD / shares
shares
Dec. 31, 2017
CAD
$ / shares
CAD / shares
shares
Dec. 31, 2016
CAD
CAD / shares
shares
Dec. 31, 2016
$ / shares
Dec. 31, 2015
CAD
CAD / shares
shares
Number of awards          
Beginning balance (in shares) | shares 6,045,014   7,164,652    
Granted (in shares) | shares 2,328,343   2,596,025    
Exercised (in shares) | shares (1,634,501)   (3,715,663)    
Ending balance (in shares) | shares 6,738,856   6,045,014   7,164,652
Exercisable (in shares) | shares 2,448,689 2,448,689      
Weighted average exercise price          
Beginning balance (CAD per share) | CAD / shares CAD 9.64   CAD 6.92    
Granted (CAD per share) | CAD / shares 12.82   10.85    
Exercised (CAD per share) | $ / shares   $ 7.81   $ 5.25  
Ending balance (CAD per share) | CAD / shares 11.18   CAD 9.64   CAD 6.92
Exercisable (CAD per share) | CAD / shares CAD 10.03 $ 10.03      
Additional Disclosures          
Outstanding shares, weighted average remaining contractual term 6 years 3 months 25 days   6 years 3 months 7 days   4 years 8 months 25 days
Granted, weighted average remaining contractual term 8 years   8 years    
Exercised shares, weighted average remaining contractual term 3 years 9 months 4 days   2 years 21 days    
Exercisable , weighted average remaining contractual term 5 years 7 months 10 days        
Beginning balance, aggregate intrinsic value | CAD CAD 10,595   CAD 28,561    
Granted, aggregate intrinsic value | CAD   0    
Exercised, aggregate intrinsic value | CAD 7,696   20,790    
Ending balance, aggregate intrinsic value | CAD 19,380   CAD 10,595   CAD 28,561
Exercisable , aggregate intrinsic value | CAD CAD 9,473,719 $ 9,473,719