Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	12 Months Ended
	Dec. 31, 2017 T
Coal sales
Derivative [Line Items]
Derivatives Held	1,865
Coal purchases
Derivative [Line Items]
Derivatives Held	747
2018 | Coal sales
Derivative [Line Items]
Derivatives Held	1,706
2018 | Coal purchases
Derivative [Line Items]
Derivatives Held	747
2019 | Coal sales
Derivative [Line Items]
Derivatives Held	159
2019 | Coal purchases
Derivative [Line Items]
Derivatives Held	0