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Share-based compensation - Range of exercise prices in SARs and TSARs (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 shares $ / shares | Dec. 31, 2019 shares $ / shares | Dec. 31, 2018 shares $ / shares
SARs |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 2 years 7 months 2 days |  |
Number of units outstanding (in shares) | shares | 840,772 | 857,407 | 896,883
Weighted average exercise price of units outstanding (in usd per unit) | $ 50.61 | $ 52.02 | $ 51.27
Number of units exercisable (in shares) | shares | 687,211 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 53.16 |  |
SARs | $29.27 to $35.51 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 3 years 7 months 13 days |  |
Number of units outstanding (in shares) | shares | 260,071 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 32.62 |  |
Number of units exercisable (in shares) | shares | 163,911 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 34.59 |  |
SARs | $29.27 to $35.51 | Bottom of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 29.27 |  |
SARs | $29.27 to $35.51 | Top of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 35.51 |  |
SARs | $45.40 to $50.17 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 3 years 1 month 9 days |  |
Number of units outstanding (in shares) | shares | 125,784 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 50.15 |  |
Number of units exercisable (in shares) | shares | 125,784 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 50.15 |  |
SARs | $45.40 to $50.17 | Bottom of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 45.40 |  |
SARs | $45.40 to $50.17 | Top of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 50.17 |  |
SARs | $54.65 to $78.59 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 1 year 10 months 9 days |  |
Number of units outstanding (in shares) | shares | 454,917 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 61.01 |  |
Number of units exercisable (in shares) | shares | 397,516 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 61.77 |  |
SARs | $54.65 to $78.59 | Bottom of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 54.65 |  |
SARs | $54.65 to $78.59 | Top of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 78.59 |  |
TSARs |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 3 years 11 months 26 days |  |
Number of units outstanding (in shares) | shares | 2,340,490 | 1,661,327 | 1,447,301
Weighted average exercise price of units outstanding (in usd per unit) | $ 45.43 | $ 52.55 | $ 51.24
Number of units exercisable (in shares) | shares | 1,278,481 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 52.51 |  |
TSARs | $29.27 to $35.51 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 5 years 7 days |  |
Number of units outstanding (in shares) | shares | 1,067,987 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 30.80 |  |
Number of units exercisable (in shares) | shares | 306,937 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 34.59 |  |
TSARs | $29.27 to $35.51 | Bottom of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 29.27 |  |
TSARs | $29.27 to $35.51 | Top of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 35.51 |  |
TSARs | $45.40 to $50.17 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 3 years 3 months 29 days |  |
Number of units outstanding (in shares) | shares | 311,184 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 49.84 |  |
Number of units exercisable (in shares) | shares | 296,637 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 50.05 |  |
TSARs | $45.40 to $50.17 | Bottom of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 45.40 |  |
TSARs | $45.40 to $50.17 | Top of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 50.17 |  |
TSARs | $54.65 to $78.59 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | 3 years 21 days |  |
Number of units outstanding (in shares) | shares | 961,319 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 60.27 |  |
Number of units exercisable (in shares) | shares | 674,907 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 61.74 |  |
TSARs | $54.65 to $78.59 | Bottom of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 54.65 |  |
TSARs | $54.65 to $78.59 | Top of range |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 78.59 |  |

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