Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	6 Months Ended
	Jun. 30, 2021 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	568
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	154
2021 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	535
2021 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	121
2022 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	33
2022 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	33