Other Short-Term and Long-Term Obligations (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Federal Coal Leases
Carrying Value	$ 186,119	$ 186,119
Fair Value	211,446	208,803
Future payments on federal coal leases
2013		79,027
2014		69,407
2015		69,407
Total		217,841
Less: imputed interest		31,722
Total principal payments	186,119	186,119
Less: current portion	63,191	63,191
Long term federal coal leases payable	122,928	122,928
May 1, 2009 - 2013
Federal Coal Leases
Annual Payment	9,620
Imputed Interest Rate (as a percent)	8.70%
Carrying Value	8,852	8,852
Fair Value	9,602	9,532
Future payments on federal coal leases
Total principal payments	8,852	8,852
July 1, 2011 - 2015
Federal Coal Leases
Annual Payment	59,545
Imputed Interest Rate (as a percent)	8.50%
Carrying Value	152,078	152,078
Fair Value	173,407	171,075
Future payments on federal coal leases
Total principal payments	152,078	152,078
September 1, 2011 - 2015
Federal Coal Leases
Annual Payment	9,862
Imputed Interest Rate (as a percent)	8.50%
Carrying Value	25,189	25,189
Fair Value	28,437	28,196
Future payments on federal coal leases
Total principal payments	$ 25,189	$ 25,189