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Asset Impairment (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2020
Coal lands and mineral rights $ 33,197  
Plant and equipment 43,197  
Deferred development 50,527  
Equity investment 36,167  
Asset impairment charges 163,088 $ 163,088
Knight Hawk    
Equity investment 36,200  
PRB | Coal Creek Mine    
Asset impairment charges 51,800  
Other Thermal | Viper Mine    
Asset impairment charges 33,500  
Other Thermal | West Elk Mine    
Asset impairment charges $ 41,600