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Significant accounting policies - Share-based compensation (Details) - Performance shares | 12 Months Ended
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Dec. 31, 2017
Grants prior to 2014 | Bottom of range |
Disclosure of terms and conditions of share-based payment arrangement [line items] |
Share-based compensation arrangement vesting rights, percentage | 50.00%
Grants prior to 2014 | Top of range |
Disclosure of terms and conditions of share-based payment arrangement [line items] |
Share-based compensation arrangement vesting rights, percentage | 120.00%
Grants subsequent to 2014 | Bottom of range |
Disclosure of terms and conditions of share-based payment arrangement [line items] |
Share-based compensation arrangement vesting rights, percentage | 25.00%
Grants subsequent to 2014 | Top of range |
Disclosure of terms and conditions of share-based payment arrangement [line items] |
Share-based compensation arrangement vesting rights, percentage | 150.00%

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage1 + References No definition available. + Details
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- Details
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- Details
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- Details
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- Details
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- Details
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