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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Asset Retirement Obligation Rollforward) (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2019
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
ARO liability balance, December 31, 2018 $ 239,865
Accretion expense 2,220
Settlements (6,068)
ARO liability balance, March 31, 2019 $ 236,017