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Shareholders' equity - Stock-based payments - Exercise price (Details)
 12 Months Ended
Dec. 31, 2019
EquityInstruments
$ / shares
Dec. 31, 2018
EquityInstruments
Dec. 31, 2017
EquityInstruments
Shareholders' equity      
Options outstanding 836,427 1,015,409 955,603
Options exercisable 596,402 710,064  
Weighted average life 6 years 10 months 10 days    
C$33.12      
Shareholders' equity      
Exercise price | $ / shares $ 33.12    
Options outstanding 1,500    
Options exercisable 1,500    
Weighted average life 10 months 24 days    
C$40.87      
Shareholders' equity      
Exercise price | $ / shares $ 40.87    
Options outstanding 38,076    
Options exercisable 38,076    
Weighted average life 3 years 11 months 12 days    
C$42.43      
Shareholders' equity      
Exercise price | $ / shares $ 42.43    
Options outstanding 2,000    
Options exercisable 2,000    
Weighted average life 2 years 3 months    
C$46.17      
Shareholders' equity      
Exercise price | $ / shares $ 46.17    
Options outstanding 90,000    
Options exercisable 90,000    
Weighted average life 3 years 7 months 17 days    
C$55.58      
Shareholders' equity      
Exercise price | $ / shares $ 55.58    
Options outstanding 27,397    
Options exercisable 27,397    
Weighted average life 2 years 11 months 12 days    
C$58.67      
Shareholders' equity      
Exercise price | $ / shares $ 58.67    
Options outstanding 35,000    
Options exercisable 35,000    
Weighted average life 5 years 7 months 21 days    
C$59.52      
Shareholders' equity      
Exercise price | $ / shares $ 59.52    
Options outstanding 53,376    
Options exercisable 53,376    
Weighted average life 4 years 11 months 12 days    
C$65.76      
Shareholders' equity      
Exercise price | $ / shares $ 65.76    
Options outstanding 65,258    
Options exercisable 65,258    
Weighted average life 5 years 11 months 12 days    
C$75.45      
Shareholders' equity      
Exercise price | $ / shares $ 75.45    
Options outstanding 178,936    
Options exercisable 178,936    
Weighted average life 6 years 11 months 12 days    
C$88.76      
Shareholders' equity      
Exercise price | $ / shares $ 88.76    
Options outstanding 45,082    
Options exercisable 13,261    
Weighted average life 8 years 7 months 21 days    
C$94.57      
Shareholders' equity      
Exercise price | $ / shares $ 94.57    
Options outstanding 98,562    
Options exercisable 27,835    
Weighted average life 8 years 11 months 12 days    
C$100.10      
Shareholders' equity      
Exercise price | $ / shares $ 100.10    
Options outstanding 96,359    
Options exercisable 63,763    
Weighted average life 7 years 11 months 12 days    
C$129.32      
Shareholders' equity      
Exercise price | $ / shares $ 129.32    
Options outstanding 104,881    
Weighted average life 9 years 11 months 12 days