Federal Coal Lease Obligations (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Long Term Contractual Obligation Maturities Annual Payment [Line Items]
Carrying Value	$ 63,970	$ 63,970
Fair Value	69,026	68,712
July 1, 2011 - 2015
Long Term Contractual Obligation Maturities Annual Payment [Line Items]
Annual Payment	59,545	59,545
Imputed Interest Rate (as a percent)	8.50%	8.50%
Carrying Value	54,880	54,880
Fair Value	59,246	58,976
September 1, 2011 - 2015
Long Term Contractual Obligation Maturities Annual Payment [Line Items]
Annual Payment	9,862	9,862
Imputed Interest Rate (as a percent)	8.50%	8.50%
Carrying Value	9,090	9,090
Fair Value	$ 9,780	$ 9,736