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Derivatives (Schedule of Price Risk Derivatives) (Details)
T in Thousands
6 Months Ended
Jun. 30, 2020
T
Coal sales  
Derivative [Line Items]  
Derivatives held (in tons) 542
Coal purchases  
Derivative [Line Items]  
Derivatives held (in tons) 377
2020 | Coal sales  
Derivative [Line Items]  
Derivatives held (in tons) 542
2020 | Coal purchases  
Derivative [Line Items]  
Derivatives held (in tons) 377