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Share-based compensation - Range of exercise prices in stock options (Details)
 12 Months Ended
Dec. 31, 2019
shares
$ / shares
Dec. 31, 2018
shares
$ / shares
Dec. 31, 2017
shares
$ / shares
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Weighted average remaining contractual life (years) 2 years 8 months 12 days    
Number of stock options outstanding (in shares) | shares 200,631 198,221 262,535
Weighted average exercise price (in usd per option) $ 49.07 $ 48.55 $ 45.09
Number of stock options exercisable (in shares) | shares 170,551    
Weighted average exercise price, exercisable options (in usd per option) $ 48.19    
$25.97 to $35.51      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Weighted average remaining contractual life (years) 3 years 2 months 1 day    
Number of stock options outstanding (in shares) | shares 53,767    
Weighted average exercise price (in usd per option) $ 34.59    
Number of stock options exercisable (in shares) | shares 53,767    
Weighted average exercise price, exercisable options (in usd per option) $ 34.59    
$25.97 to $35.51 | Minimum      
Disclosure of range of exercise prices of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) 25.97    
$25.97 to $35.51 | Maximum      
Disclosure of range of exercise prices of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) $ 35.51    
$38.24 to $50.17      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Weighted average remaining contractual life (years) 1 year 11 months 16 days    
Number of stock options outstanding (in shares) | shares 56,554    
Weighted average exercise price (in usd per option) $ 43.56    
Number of stock options exercisable (in shares) | shares 47,751    
Weighted average exercise price, exercisable options (in usd per option) $ 42.35    
$38.24 to $50.17 | Minimum      
Disclosure of range of exercise prices of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) 38.24    
$38.24 to $50.17 | Maximum      
Disclosure of range of exercise prices of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) $ 50.17    
$54.65 to $78.59      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Weighted average remaining contractual life (years) 2 years 10 months 17 days    
Number of stock options outstanding (in shares) | shares 90,310    
Weighted average exercise price (in usd per option) $ 61.14    
Number of stock options exercisable (in shares) | shares 69,033    
Weighted average exercise price, exercisable options (in usd per option) $ 62.82    
$54.65 to $78.59 | Minimum      
Disclosure of range of exercise prices of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) 54.65    
$54.65 to $78.59 | Maximum      
Disclosure of range of exercise prices of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) $ 78.59