Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	3 Months Ended
	Mar. 31, 2019 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	2,540
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	1,224
2019 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	2,196
2019 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	1,131
2020 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	344
2020 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	93