Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	9 Months Ended
	Sep. 30, 2021 T
Coal sales.
Derivative [Line Items]
Derivatives held (in tons)	371
Coal purchases.
Derivative [Line Items]
Derivatives held (in tons)	64
2021 | Coal sales.
Derivative [Line Items]
Derivatives held (in tons)	338
2021 | Coal purchases.
Derivative [Line Items]
Derivatives held (in tons)	31
2022 | Coal sales.
Derivative [Line Items]
Derivatives held (in tons)	33
2022 | Coal purchases.
Derivative [Line Items]
Derivatives held (in tons)	33