Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	12 Months Ended
	Dec. 31, 2018 T
Coal sales
Derivative [Line Items]
Derivatives Held	2,694
Coal purchases
Derivative [Line Items]
Derivatives Held	1,461
2019 | Coal sales
Derivative [Line Items]
Derivatives Held	2,601
2019 | Coal purchases
Derivative [Line Items]
Derivatives Held	1,368
2020 | Coal sales
Derivative [Line Items]
Derivatives Held	93
2020 | Coal purchases
Derivative [Line Items]
Derivatives Held	93