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Commitments and contingencies - Minimum purchase contract commitment (Details) - Take-or-pay purchase contracts $ in Thousands | Dec. 31, 2018 USD ($)
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2019 |
Disclosure of contingent liabilities [line items] |
Minimum estimated commitment | $ 456,804
2020 |
Disclosure of contingent liabilities [line items] |
Minimum estimated commitment | 369,401
2021 |
Disclosure of contingent liabilities [line items] |
Minimum estimated commitment | 371,345
2022 |
Disclosure of contingent liabilities [line items] |
Minimum estimated commitment | 340,595
2023 |
Disclosure of contingent liabilities [line items] |
Minimum estimated commitment | 319,509
Thereafter |
Disclosure of contingent liabilities [line items] |
Minimum estimated commitment | $ 1,502,356

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