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Share-based compensation - Changes in SARs and TSARs (Details) | 12 Months Ended
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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

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