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Significant accounting policies - Share-based compensation (Details)	12 Months Ended
	Dec. 31, 2019
Performance shares | Tranche 2 | Bottom of range
Disclosure of terms and conditions of share-based payment arrangement [line items]
Share-based compensation arrangement vesting rights, percentage	25.00%
Performance shares | Tranche 2 | Top of range
Disclosure of terms and conditions of share-based payment arrangement [line items]
Share-based compensation arrangement vesting rights, percentage	150.00%
Performance Share Units (PSU) New Plan
Disclosure of terms and conditions of share-based payment arrangement [line items]
Vesting period	3 years
Performance Share Units (PSU) New Plan | Tranche 2 | Bottom of range
Disclosure of terms and conditions of share-based payment arrangement [line items]
Share-based compensation arrangement vesting rights, percentage	0.00%
Performance Share Units (PSU) New Plan | Tranche 2 | Top of range
Disclosure of terms and conditions of share-based payment arrangement [line items]
Share-based compensation arrangement vesting rights, percentage	200.00%

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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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