Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	9 Months Ended
	Sep. 30, 2017 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	813
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	437
2017 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	267
2017 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	252
2018 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	546
2018 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	185