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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Depreciation and Depletion Expense) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Fixed Assets And Asset Retirement Obligations [Abstract]        
Depreciation expense $ 62,499 $ 77,228 $ 206,830 $ 222,106
Depletion expense $ 604 $ 1,204 $ 2,408 $ 3,488