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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Narrative) (Details) - USD ($)
$ in Thousands
Mar. 31, 2022
Dec. 31, 2021
Asset Retirement Obligations Details [Line Items]    
Asset retirement obligation balance $ 222,892 $ 220,906
Accrued Liabilities    
Asset Retirement Obligations Details [Line Items]    
Asset retirement obligation balance $ 35,300 $ 36,300