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Significant accounting policies - Share-based compensation (Details) - Performance shares	12 Months Ended
	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.

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No definition available.

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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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