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Commitments and contingencies - Minimum purchase contract commitment (Details) - Take-or-pay purchase contracts
$ in Thousands
Dec. 31, 2017
USD ($)
2018  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment $ 473,927
2019  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 371,167
2020  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 306,400
2021  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 308,338
2022  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment 245,989
Thereafter  
Disclosure of contingent liabilities [line items]  
Minimum estimated commitment $ 1,300,609