Other Short-Term and Long-Term Obligations (Details 2) (USD $)	3 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 May 1, 2009 - 2013	Dec. 31, 2012 May 1, 2009 - 2013	Jun. 30, 2013 July 1, 2011 - 2015	Dec. 31, 2012 July 1, 2011 - 2015	Jun. 30, 2013 September 1, 2011 - 2015	Dec. 31, 2012 September 1, 2011 - 2015
Federal Coal Leases
Annual Payment			$ 9,620,000		$ 59,545,000		$ 9,862,000
Imputed Interest Rate (as a percent)			8.70%		8.50%		8.50%
Carrying Value	177,267,000	186,119,000		8,852,000	152,078,000	152,078,000	25,189,000	25,189,000
Fair Value	203,263,000	208,803,000		9,532,000	174,744,000	171,075,000	28,519,000	28,196,000
Future payments on federal coal leases
2013	69,407,000
2014	69,407,000
2015	69,407,000
Total	208,221,000
Less: imputed interest	30,954,000
Total principal payments	177,267,000	186,119,000		8,852,000	152,078,000	152,078,000	25,189,000	25,189,000
Less: current portion	54,339,000	63,191,000
Long term federal coal leases payable	122,928,000	122,928,000
Capital Equipment Lease Obligations
Base rate on interest on leases	one-month LIBOR
Margin over base rate on interest on leases (as a percent)	1.95%
Interest rate (as a percent)	2.14%
Gross value of property, plant, and equipment under capital leases	7,600,000
Accumulated depreciation of property, plant, and equipment under capital leases	200,000
Capital equipment lease obligation payments
2013	621,000
2014	1,225,000
2015	1,201,000
2016	1,178,000
2017	1,155,000
Thereafter	2,694,000
Total	8,074,000
Less: interest	563,000
Total principal payments	7,511,000
Less: current portion	1,086,000
Long-term capital equipment lease obligations	$ 6,425,000