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Share-based compensation - Changes in SARs and TSARs (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
SARs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of outstanding units, beginning of period | shares 1,511,485 1,259,208
Number of outstanding units granted | shares 167,600 375,500
Number of outstanding units exercised | shares (213,207) (73,291)
Number of outstanding units cancelled | shares (10,801) (49,932)
Number of outstanding units expired | shares (5,000)  
Number of outstanding units, end of period | shares 1,450,077 1,511,485
Exercise price of outstanding units, beginning of period (in usd per unit) | $ $ 42.68 $ 44.48
Exercise price of outstanding units granted (in usd per unit) | $ 50.15 34.59
Exercise price of outstanding units exercised (in usd per unit) | $ 32.03 27.43
Exercise price of outstanding units cancelled (in usd per unit) | $ 50.18 49.77
Exercise price of outstanding units expired (in usd per unit) | $ 25.22  
Exercise price of outstanding units, end of period (in usd per unit) | $ $ 45.11 $ 42.68
TSARs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of outstanding units, beginning of period | shares 2,416,111 2,108,965
Number of outstanding units granted | shares 340,200 574,600
Number of outstanding units exercised | shares (710,616) (212,505)
Number of outstanding units cancelled | shares (2,200) (54,949)
Number of outstanding units expired | shares 0  
Number of outstanding units, end of period | shares 2,043,495 2,416,111
Exercise price of outstanding units, beginning of period (in usd per unit) | $ $ 42.10 $ 42.73
Exercise price of outstanding units granted (in usd per unit) | $ 50.17 34.59
Exercise price of outstanding units exercised (in usd per unit) | $ 32.98 25.38
Exercise price of outstanding units cancelled (in usd per unit) | $ 34.59 52.55
Exercise price of outstanding units expired (in usd per unit) | $ 0.00  
Exercise price of outstanding units, end of period (in usd per unit) | $ $ 46.62 $ 42.10