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Debt - J. Aron Forward Sale (Details) - USD ($)
$ in Thousands
8 Months Ended 9 Months Ended
May 08, 2017
Dec. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Debt Instrument [Line Items]          
Principal amount of long-term debt     $ 353,457   $ 400,420
Principal payments - 2017     292,684 $ 137,791  
Supply and Offtake Agreements          
Debt Instrument [Line Items]          
Agreement extension term 1 year        
Forward sale liability $ 30,000        
Debt instrument, interest rate 7.00%        
J. Aron Forward Sale          
Debt Instrument [Line Items]          
Principal amount of long-term debt     $ 27,855   $ 0
Principal payments - 2018 $ 7,000        
Principal payments - 2019 7,500        
Principal payments - 2020 8,100        
Principal payments - 2021 $ 3,500        
Scenario, Forecast | J. Aron Forward Sale          
Debt Instrument [Line Items]          
Principal payments - 2017   $ 3,900