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Commitments and contingencies - Minimum purchase contract commitment (Details) - Take-or-pay purchase contracts
$ in Thousands
Dec. 31, 2018
USD ($)
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