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Fixed Assets, Mineral Leaseholds, and Asset Retirement Obligations (Depreciation and Depletion Expense) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Fixed Assets And Asset Retirement Obligations [Abstract]        
Depreciation expense $ 75,089 $ 73,206 $ 144,331 $ 144,878
Depletion expense $ 841 $ 965 $ 1,804 $ 2,284