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

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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 2020-01-01 -Paragraph 47 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2020-en-r&anchor=para_47_b&doctype=Standard -URIDate 2020-01-01 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Name IFRS -Number 2 -IssueDate 2020-01-01 -Paragraph 45 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2020-en-r&anchor=para_45&doctype=Standard -URIDate 2020-01-01 + Details
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