Other Long-Term Obligations (Details) (USD $)			0 Months Ended	1 Months Ended	0 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	May 11, 2011 WAII North T item	May 31, 2011 WAII North	Jun. 15, 2011 WAII South item T	Jun. 30, 2011 WAII South	Jun. 30, 2011 Mineral rights
Other Long-Term Obligations
Federal coal lease obligations, current	$ 63,191,000	$ 102,198,000
Federal coal lease obligations, noncurrent	122,928,000	186,119,000
Total principal payments	186,119,000	288,317,000
Federal Coal Lease Obligations
Bid amount for lease sale			297,700,000		49,300,000
Bid amount for lease sale (in dollars per ton)			0.85		0.875
Quantity of BLM's estimate (in tons)			350,000,000		56,000,000
Payment for acquisition of lease				59,500,000		9,900,000
Number of additional payments payable annually for acquisition of lease			4		4
Property, plant and equipment, net	1,678,294,000	1,350,135,000					293,900,000
Discounted amount of future lease payment obligations							$ 224,500,000