Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	Jun. 30, 2017 T
Coal sales
Derivative [Line Items]
Derivatives Held	371
Coal purchases
Derivative [Line Items]
Derivatives Held	363
2017 | Coal sales
Derivative [Line Items]
Derivatives Held	371
2017 | Coal purchases
Derivative [Line Items]
Derivatives Held	363
2018 | Coal sales
Derivative [Line Items]
Derivatives Held	0
2018 | Coal purchases
Derivative [Line Items]
Derivatives Held	0