Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	9 Months Ended
	Sep. 30, 2018 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	2,266
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	402
2018 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	646
2018 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	284
2019 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	1,620
2019 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	118