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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Forecast of Accretion Expense) (Details)
$ in Thousands
Mar. 31, 2019
USD ($)
Fixed Assets And Asset Retirement Obligations [Abstract]  
Remainder of 2019 $ 7,660
2020 10,997
2021 9,313
2022 9,892
2023 $ 10,586