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Derivatives (Schedule of Price Risk Derivatives) (Details)
T in Thousands
9 Months Ended
Sep. 30, 2019
T
Coal sales  
Derivative [Line Items]  
Derivatives held (in tons) 1,275
Coal purchases  
Derivative [Line Items]  
Derivatives held (in tons) 722
2019 | Coal sales  
Derivative [Line Items]  
Derivatives held (in tons) 607
2019 | Coal purchases  
Derivative [Line Items]  
Derivatives held (in tons) 385
2020 | Coal sales  
Derivative [Line Items]  
Derivatives held (in tons) 668
2020 | Coal purchases  
Derivative [Line Items]  
Derivatives held (in tons) 337