Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	Mar. 31, 2017 T
Coal sales [Member]
Derivative [Line Items]
Derivatives Held	405
Coal purchases [Member]
Derivative [Line Items]
Derivatives Held	360
2017 [Member] | Coal sales [Member]
Derivative [Line Items]
Derivatives Held	405
2017 [Member] | Coal purchases [Member]
Derivative [Line Items]
Derivatives Held	360
2018 [Member] | Coal sales [Member]
Derivative [Line Items]
Derivatives Held	0
2018 [Member] | Coal purchases [Member]
Derivative [Line Items]
Derivatives Held	0