Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	6 Months Ended
	Jun. 30, 2019 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	2,029
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	997
2019 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	1,540
2019 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	772
2020 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	489
2020 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	225