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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Asset Retirement Obligation Rollforward) (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2022
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
ARO liability balance, December 31, 2021 $ 220,906
Accretion expense 6,871
Changes in estimate 2,383
Settlements (9,798)
ARO liability balance, June 30, 2022 $ 220,362