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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Asset Retirement Obligation Rollforward) (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2020
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
ARO liability balance, December 31, 2019 $ 175,081
Accretion expense 4,510
Changes in estimate 560
Settlements (9,851)
ARO liability balance, June 30, 2020 $ 170,300