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 |