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Share-based compensation - Range of exercise prices in SARs and TSARs (Details) | Dec. 31, 2017 USD ($) year shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 USD ($) shares
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SARs |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | year | 3.61 |  |
Number of units outstanding | shares | 1,450,077 | 1,511,485 | 1,259,208
Weighted average exercise price of units outstanding (in usd per unit) | $ 45.11 | $ 42.68 | $ 44.48
Number of units exercisable | shares | 955,316 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 45.93 |  |
SARs | $25.97 to $35.51 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | year | 3.36 |  |
Number of units outstanding | shares | 585,317 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 32.94 |  |
Number of units exercisable | shares | 345,629 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 31.79 |  |
SARs | $25.97 to $35.51 | Minimum |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 25.97 |  |
SARs | $25.97 to $35.51 | Maximum |  |  |
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 | $38.24 to $73.13 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | year | 3.77 |  |
Number of units outstanding | shares | 864,760 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 53.35 |  |
Number of units exercisable | shares | 609,687 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 53.94 |  |
SARs | $38.24 to $73.13 | Minimum |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 38.24 |  |
SARs | $38.24 to $73.13 | Maximum |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 73.13 |  |
TSARs |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | year | 4.03 |  |
Number of units outstanding | shares | 2,043,495 | 2,416,111 | 2,108,965
Weighted average exercise price of units outstanding (in usd per unit) | $ 46.62 | $ 42.10 | $ 42.73
Number of units exercisable | shares | 1,195,707 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 48.44 |  |
TSARs | $25.97 to $35.51 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | year | 3.94 |  |
Number of units outstanding | shares | 779,194 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 33.67 |  |
Number of units exercisable | shares | 401,049 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 32.80 |  |
TSARs | $25.97 to $35.51 | Minimum |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 25.97 |  |
TSARs | $25.97 to $35.51 | Maximum |  |  |
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 | $38.24 to $73.13 |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average remaining contractual life (years) | year | 4.09 |  |
Number of units outstanding | shares | 1,264,301 |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 54.60 |  |
Number of units exercisable | shares | 794,658 |  |
Weighted average exercise price of units exercisable (in usd per unit) | $ 56.33 |  |
TSARs | $38.24 to $73.13 | Minimum |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | 38.24 |  |
TSARs | $38.24 to $73.13 | Maximum |  |  |
Disclosure of terms and conditions of share-based payment arrangement [line items] |  |  |
Weighted average exercise price of units outstanding (in usd per unit) | $ 73.13 |  |

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