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Share-based compensation - Changes in SARs and TSARs (Details)
 12 Months Ended
Dec. 31, 2019
shares
$ / shares
Dec. 31, 2018
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 896,883 1,450,077
Number of outstanding units granted (in shares) | shares 29,320 141,300
Number of outstanding units exercised (in shares) | shares (39,662) (669,931)
Number of outstanding units cancelled (in shares) | shares (29,134) (16,582)
Number of outstanding units expired (in shares) | shares   (7,981)
Number of outstanding units, end of period (in shares) | shares 857,407 896,883
Exercise price of outstanding units, beginning of period (in usd per unit) | $ / shares $ 51.27 $ 45.11
Exercise price of outstanding units granted (in usd per unit) | $ / shares 57.60 55.28
Exercise price of outstanding units exercised (in usd per unit) | $ / shares 37.25 39.00
Exercise price of outstanding units cancelled (in usd per unit) | $ / shares 54.72 53.12
Exercise price of outstanding units expired (in usd per unit) | $ / shares   28.74
Exercise price of outstanding units, end of period (in usd per unit) | $ / shares $ 52.02 $ 51.27
TSARs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of outstanding units, beginning of period (in shares) | shares 1,447,301 2,043,495
Number of outstanding units granted (in shares) | shares 294,680 330,400
Number of outstanding units exercised (in shares) | shares (45,769) (918,327)
Number of outstanding units cancelled (in shares) | shares (34,885) (8,267)
Number of outstanding units expired (in shares) | shares   0
Number of outstanding units, end of period (in shares) | shares 1,661,327 1,447,301
Exercise price of outstanding units, beginning of period (in usd per unit) | $ / shares $ 51.24 $ 46.62
Exercise price of outstanding units granted (in usd per unit) | $ / shares 56.70 55.37
Exercise price of outstanding units exercised (in usd per unit) | $ / shares 37.08 42.48
Exercise price of outstanding units cancelled (in usd per unit) | $ / shares 53.38 47.25
Exercise price of outstanding units expired (in usd per unit) | $ / shares   0.00
Exercise price of outstanding units, end of period (in usd per unit) | $ / shares $ 52.55 $ 51.24