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Share-based compensation - Range of exercise prices in stock options (Details)
 12 Months Ended
Dec. 31, 2020
shares
$ / shares
Dec. 31, 2019
shares
$ / shares
Dec. 31, 2018
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 6 months 10 days    
Number of stock options outstanding (in shares) | shares 173,251 200,631 198,221
Weighted average exercise price (in usd per option) $ 49.44 $ 49.07 $ 48.55
Number of stock options exercisable (in shares) | shares 146,303    
Weighted average exercise price, exercisable options (in usd per option) $ 51.06    
$29.27 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 1 month 20 days    
Number of stock options outstanding (in shares) | shares 63,307    
Weighted average exercise price (in usd per option) $ 33.29    
Number of stock options exercisable (in shares) | shares 47,867    
Weighted average exercise price, exercisable options (in usd per option) $ 34.59    
$29.27 to $35.51 | Bottom of range      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) 29.27    
$29.27 to $35.51 | Top of range      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) $ 35.51    
$45.40 to $50.17      
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 24,034    
Weighted average exercise price (in usd per option) $ 50.17    
Number of stock options exercisable (in shares) | shares 24,034    
Weighted average exercise price, exercisable options (in usd per option) $ 50.17    
$45.40 to $50.17 | Bottom of range      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) 45.40    
$45.40 to $50.17 | Top of range      
Disclosure of number and weighted average remaining contractual life 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) 1 year 10 months 20 days    
Number of stock options outstanding (in shares) | shares 85,910    
Weighted average exercise price (in usd per option) $ 61.13    
Number of stock options exercisable (in shares) | shares 74,402    
Weighted average exercise price, exercisable options (in usd per option) $ 61.94    
$54.65 to $78.59 | Bottom of range      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) 54.65    
$54.65 to $78.59 | Top of range      
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]      
Exercise price of outstanding share options (usd per share) $ 78.59