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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Asset Retirement Obligation Rollforward) (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2019
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
ARO liability balance, December 31, 2018 $ 239,865
Accretion expense 6,825
Changes in estimate (14,377)
Settlements (36,028)
ARO liability balance, September 30, 2019 $ 196,285