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Commitments and contingencies - Minimum purchase contract commitment (Details) - Take-or-pay purchase contracts
$ in Thousands
Dec. 31, 2019
USD ($)
2020  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment $ 458,859
2021  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 398,267
2022  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 356,268
2023  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 356,861
2024  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 375,281
Thereafter  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment $ 1,386,390