Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands	Dec. 31, 2016 T
Coal Sales [Member]
Derivative [Line Items]
Derivatives Held	540
Coal Purchases [Member]
Derivative [Line Items]
Derivatives Held	480
Year One [Member] | Coal Sales [Member]
Derivative [Line Items]
Derivatives Held	540
Year One [Member] | Coal Purchases [Member]
Derivative [Line Items]
Derivatives Held	480
Year Two [Member] | Coal Sales [Member]
Derivative [Line Items]
Derivatives Held	0
Year Two [Member] | Coal Purchases [Member]
Derivative [Line Items]
Derivatives Held	0