Other Long-Term Obligations (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Federal Coal Leases
Carrying Value	$ 186,119,000	$ 288,317,000
Fair Value	208,803,000	318,473,000
Future payments on federal coal leases
2013	79,027,000,000
2014	69,407,000,000
2015	69,407,000,000
Total	217,841,000,000
Less: imputed interest	31,722,000,000
Total principal payments	186,119,000	288,317,000
Less: current portion	63,191,000	102,198,000
Long term federal coal leases payable	122,928,000	186,119,000
Imputed interest
Imputed interest	20,406,000	19,619,000	11,673,000
Other long-term obligations
Purchase obligations, total	2,661,000	6,567,000
Interest rate, low end of range (as a percent)	6.00%	6.00%
Interest rate, high end of range (as a percent)	8.00%	8.00%
August 1, 2008 - 2012
Federal Coal Leases
Annual Payment	50,160,000
Imputed Interest Rate (as a percent)	7.50%
Carrying Value		46,661,000
Fair Value		48,867,000
Future payments on federal coal leases
Total principal payments		46,661,000
May 1, 2009 - 2013
Federal Coal Leases
Annual Payment	9,620,000
Imputed Interest Rate (as a percent)	8.70%
Carrying Value	8,852,000	16,998,000
Fair Value	9,532,000	18,517,000
Future payments on federal coal leases
Total principal payments	8,852,000	16,998,000
July 1, 2011 - 2015
Federal Coal Leases
Annual Payment	59,545,000
Imputed Interest Rate (as a percent)	8.50%
Carrying Value	152,078,000	192,892,000
Fair Value	171,075,000	215,796,000
Future payments on federal coal leases
Total principal payments	152,078,000	192,892,000
September 1, 2011 - 2015
Federal Coal Leases
Annual Payment	9,862,000
Imputed Interest Rate (as a percent)	8.50%
Carrying Value	25,189,000	31,766,000
Fair Value	28,196,000	35,293,000
Future payments on federal coal leases
Total principal payments	$ 25,189,000	$ 31,766,000