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Shareholders' equity - Stock-based payments - Exercise price (Details)
Dec. 31, 2018
USD ($)
EquityInstruments
Y
Dec. 31, 2017
EquityInstruments
Dec. 31, 2017
USD ($)
Dec. 31, 2016
EquityInstruments
Shareholders' equity        
Options outstanding 1,015,409 955,603 955,603 1,304,328
Options exercisable 710,064 617,539    
Weighted average life | Y 7.00      
C$31.39        
Shareholders' equity        
Exercise price | $ $ 31.39      
Options outstanding 10,000      
Options exercisable 10,000      
Weighted average life | Y 1.39      
C$31.45        
Shareholders' equity        
Exercise price | $ $ 31.45      
Options outstanding 38,000      
Options exercisable 38,000      
Weighted average life | Y 1.71      
C$33.12        
Shareholders' equity        
Exercise price | $ $ 33.12      
Options outstanding 1,500      
Options exercisable 1,500      
Weighted average life | Y 1.90      
C$33.20        
Shareholders' equity        
Exercise price | $ $ 33.20      
Options outstanding 1,000      
Options exercisable 1,000      
Weighted average life | Y 1.98      
C$40.87        
Shareholders' equity        
Exercise price | $ $ 40.87      
Options outstanding 52,786      
Options exercisable 52,786      
Weighted average life | Y 4.94      
C$42.43        
Shareholders' equity        
Exercise price | $ $ 42.43      
Options outstanding 3,000      
Options exercisable 3,000      
Weighted average life | Y 3.25      
C$42.67        
Shareholders' equity        
Exercise price | $ $ 42.67      
Options outstanding 2,500      
Options exercisable 2,500      
Weighted average life | Y 2.92      
C$46.17        
Shareholders' equity        
Exercise price | $ $ 46.17      
Options outstanding 100,000      
Options exercisable 100,000      
Weighted average life | Y 4.63      
C$55.58        
Shareholders' equity        
Exercise price | $ $ 55.58      
Options outstanding 27,397      
Options exercisable 27,397      
Weighted average life | Y 3.95      
C$58.67        
Shareholders' equity        
Exercise price | $ $ 58.67      
Options outstanding 65,000      
Options exercisable 65,000      
Weighted average life | Y 6.64      
C$59.52        
Shareholders' equity        
Exercise price | $ $ 59.52      
Options outstanding 113,914      
Options exercisable 113,914      
Weighted average life | Y 5.95      
C$65.76        
Shareholders' equity        
Exercise price | $ $ 65.76      
Options outstanding 91,635      
Options exercisable 91,635      
Weighted average life | Y 6.95      
C$75.45        
Shareholders' equity        
Exercise price | $ $ 75.45      
Options outstanding 257,064      
Options exercisable 170,736      
Weighted average life | Y 7.95      
C$88.76        
Shareholders' equity        
Exercise price | $ $ 88.76      
Options outstanding 47,732      
Weighted average life | Y 9.64      
C$94.57        
Shareholders' equity        
Exercise price | $ $ 94.57      
Options outstanding 106,092      
Weighted average life | Y 9.95      
C$100.10        
Shareholders' equity        
Exercise price | $ $ 100.10      
Options outstanding 97,789      
Options exercisable 32,596      
Weighted average life | Y 8.94