Commitments and Contingencies (Details 2) (USD $) In Thousands, unless otherwise specified			0 Months Ended
	Dec. 31, 2012	Dec. 05, 2012	Aug. 23, 2012 Western Minerals LLC item	Dec. 31, 2012 Equipment	Dec. 31, 2011 Equipment	Dec. 31, 2012 Land	Dec. 31, 2011 Land	Dec. 31, 2012 Coal purchase commitments	Dec. 31, 2011 Coal purchase commitments	Dec. 31, 2012 Transportation agreements	Dec. 31, 2011 Transportation agreements	Dec. 31, 2012 Materials and supplies	Dec. 31, 2011 Materials and supplies
Contingencies
Outstanding amount of long-term purchase commitment				$ 20,317	$ 8,637	$ 23,700	$ 23,700	$ 28,633	$ 5,652	$ 159,398	$ 135,080	$ 24,552	$ 29,641
Number of counterclaims asserted			6
Ownership interest in Decker (as a percent)	50.00%	50.00%