Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	9 Months Ended
	Sep. 30, 2020 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	288
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	238
2020 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	255
2020 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	238
2021 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	33