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Commitments and Contingencies - Future Minimum Contractual Commitments under Non-cancelable Agreements (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
Operating Leased Assets [Line Items]  
2019 $ 2,785
2020 1,870
2021 757
2022 0
2023 0
Thereafter 0
Gas Contract  
Operating Leased Assets [Line Items]  
2019 1,643
2020 1,647
2021 680
2022 0
2023 0
Thereafter 0
Office Leases  
Operating Leased Assets [Line Items]  
2019 723
2020 0
2021 0
2022 0
2023 0
Thereafter 0
Automobile Leases  
Operating Leased Assets [Line Items]  
2019 419
2020 223
2021 77
2022 0
2023 0
Thereafter $ 0