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Share-based compensation - Changes in stock options (Details)
 12 Months Ended
Dec. 31, 2019
shares
$ / shares
Dec. 31, 2018
shares
$ / shares
Share-Based Payment Arrangements [Abstract]    
Number of stock options outstanding, beginning of period (in shares) | shares 198,221 262,535
Number of stock options granted (in shares) | shares 7,410 21,900
Number of stock options exercised (in shares) | shares (2,700) (83,114)
Number of stock options cancelled (in shares) | shares (2,300) (3,100)
Number of stock options outstanding, end of period (in shares) | shares 200,631 198,221
Weighted average exercise price of stock options outstanding, beginning of period (in usd per option) | $ / shares $ 48.55 $ 45.09
Weighted average exercise price of stock options granted (in usd per option) | $ / shares 57.60 54.65
Weighted average exercise price (in usd per option) | $ / shares 31.73 38.89
Weighted average exercise price of stock options cancelled (in usd per option) | $ / shares 52.31 57.26
Weighted average exercise price of stock options outstanding, end of period (in usd per option) | $ / shares $ 49.07 $ 48.55