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Shareholders' equity - Stock-based payments - Exercise price (Details)
Dec. 31, 2017
USD ($)
EquityInstruments
Y
Dec. 31, 2016
EquityInstruments
Dec. 31, 2015
EquityInstruments
Shareholders' equity      
Options outstanding 955,603 1,304,328 1,592,480
Options exercisable 617,539 825,875  
Weighted average life | Y 7.46    
C$15.41      
Shareholders' equity      
Exercise price | $ $ 15.41    
Options outstanding 5,000    
Options exercisable 5,000    
Weighted average life | Y 0.89    
C$31.39      
Shareholders' equity      
Exercise price | $ $ 31.39    
Options outstanding 10,000    
Options exercisable 10,000    
Weighted average life | Y 2.39    
C$31.45      
Shareholders' equity      
Exercise price | $ $ 31.45    
Options outstanding 38,000    
Options exercisable 38,000    
Weighted average life | Y 2.71    
C$33.12      
Shareholders' equity      
Exercise price | $ $ 33.12    
Options outstanding 1,500    
Options exercisable 1,500    
Weighted average life | Y 2.90    
C$33.20      
Shareholders' equity      
Exercise price | $ $ 33.20    
Options outstanding 1,000    
Options exercisable 1,000    
Weighted average life | Y 2.98    
C$40.87      
Shareholders' equity      
Exercise price | $ $ 40.87    
Options outstanding 52,786    
Options exercisable 52,786    
Weighted average life | Y 5.95    
C$42.43      
Shareholders' equity      
Exercise price | $ $ 42.43    
Options outstanding 3,000    
Options exercisable 3,000    
Weighted average life | Y 4.25    
C$42.67      
Shareholders' equity      
Exercise price | $ $ 42.67    
Options outstanding 2,500    
Options exercisable 2,500    
Weighted average life | Y 3.92    
C$46.17      
Shareholders' equity      
Exercise price | $ $ 46.17    
Options outstanding 100,000    
Options exercisable 100,000    
Weighted average life | Y 5.64    
C$55.38      
Shareholders' equity      
Exercise price | $ $ 55.58    
Options outstanding 27,397    
Options exercisable 27,397    
Weighted average life | Y 4.95    
C$59.52      
Shareholders' equity      
Exercise price | $ $ 59.52    
Options outstanding 155,936    
Options exercisable 155,936    
Weighted average life | Y 6.95    
C$58.67      
Shareholders' equity      
Exercise price | $ $ 58.67    
Options outstanding 65,000    
Options exercisable 43,333    
Weighted average life | Y 7.64    
C$65.76      
Shareholders' equity      
Exercise price | $ $ 65.76    
Options outstanding 136,718    
Options exercisable 90,759    
Weighted average life | Y 7.95    
C$75.45      
Shareholders' equity      
Exercise price | $ $ 75.45    
Options outstanding 258,977    
Options exercisable 86,328    
Weighted average life | Y 8.95    
C$100.10      
Shareholders' equity      
Exercise price | $ $ 100.10    
Options outstanding 97,789    
Weighted average life | Y 9.95