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

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- Definition The number of other equity instruments (ie other than share options) granted in a share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 2 -IssueDate 2019-01-01 -Paragraph 47 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_47_b&doctype=Standard -URIDate 2019-03-27 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Name IFRS -Number 2 -IssueDate 2019-01-01 -Paragraph 45 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_45&doctype=Standard -URIDate 2019-03-27 + Details
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