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Share-based compensation - Changes in SARs and TSARs (Details)
 12 Months Ended
Dec. 31, 2020
shares
$ / shares
Dec. 31, 2019
shares
$ / shares
SARs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of outstanding units, beginning of period (in shares) | shares 857,407 896,883
Number of outstanding units granted (in shares) | shares 96,160 29,320
Number of outstanding units exercised (in shares) | shares (20,635) (39,662)
Number of outstanding units cancelled (in shares) | shares (31,660) (29,134)
Number of outstanding units expired (in shares) | shares (60,500)  
Number of outstanding units, end of period (in shares) | shares 840,772 857,407
Exercise price of outstanding units, beginning of period (in usd per unit) | $ / shares $ 52.02 $ 51.27
Exercise price of outstanding units granted (in usd per unit) | $ / shares 29.27 57.60
Exercise price of outstanding units exercised (in usd per unit) | $ / shares 34.59 37.25
Exercise price of outstanding units cancelled (in usd per unit) | $ / shares 58.13 54.72
Exercise price of outstanding units expired (in usd per unit) | $ / shares 38.24  
Exercise price of outstanding units, end of period (in usd per unit) | $ / shares $ 50.61 $ 52.02
TSARs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of outstanding units, beginning of period (in shares) | shares 1,661,327 1,447,301
Number of outstanding units granted (in shares) | shares 761,050 294,680
Number of outstanding units exercised (in shares) | shares (1,900) (45,769)
Number of outstanding units cancelled (in shares) | shares (5,967) (34,885)
Number of outstanding units expired (in shares) | shares (74,020)  
Number of outstanding units, end of period (in shares) | shares 2,340,490 1,661,327
Exercise price of outstanding units, beginning of period (in usd per unit) | $ / shares $ 52.55 $ 51.24
Exercise price of outstanding units granted (in usd per unit) | $ / shares 29.27 56.70
Exercise price of outstanding units exercised (in usd per unit) | $ / shares 34.59 37.08
Exercise price of outstanding units cancelled (in usd per unit) | $ / shares 58.38 53.38
Exercise price of outstanding units expired (in usd per unit) | $ / shares 38.24  
Exercise price of outstanding units, end of period (in usd per unit) | $ / shares $ 45.43 $ 52.55