Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	3 Months Ended
	Mar. 31, 2018 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	2,725
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	912
2018 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	1,601
2018 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	780
2019 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	1,124
2019 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	132