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Derivatives (Schedule of Price Risk Derivatives) (Details)
T in Thousands
Jun. 30, 2016
T
Coal sales [Member]  
Derivative [Line Items]  
Derivatives Held 745
Coal purchases [Member]  
Derivative [Line Items]  
Derivatives Held 165
2016 [Member] | Coal sales [Member]  
Derivative [Line Items]  
Derivatives Held 265
2016 [Member] | Coal purchases [Member]  
Derivative [Line Items]  
Derivatives Held 165
2017 [Member] | Coal sales [Member]  
Derivative [Line Items]  
Derivatives Held 480
2017 [Member] | Coal purchases [Member]  
Derivative [Line Items]  
Derivatives Held 0