Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	6 Months Ended
	Jun. 30, 2018 T
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	2,676
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	512
2018 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	1,089
2018 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	433
2019 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	1,587
2019 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	79