Derivatives (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Total Derivative Activity
Derivative financial instruments asset at the beginning of the period	$ 2,275
Total mark-to-market gains	22,754	2,275
Realized gains upon settlement	(11,244)
Derivative financial instruments asset at the end of the period	13,785	2,275
International Coal Forward Contracts | Underlying basis Tons
Risk management derivatives
Notional Amount	1,058,000	537,000
Asset	13,677	2,275
Liability	(30)
International Coal Forward Contracts | Underlying basis Tons | 2012
Risk management derivatives
Notional Amount		215,000
Asset		1,090
International Coal Forward Contracts | Underlying basis Tons | 2013
Risk management derivatives
Notional Amount	516,000	322,000
Asset	9,288	1,185
International Coal Forward Contracts | Underlying basis Tons | 2014
Risk management derivatives
Notional Amount	198,000
Asset	2,776
International Coal Forward Contracts | Underlying basis Tons | 2015
Risk management derivatives
Notional Amount	212,000
Asset	1,598
International Coal Forward Contracts | Underlying basis Tons | 2016
Risk management derivatives
Notional Amount	132,000
Asset	15
Liability	(30)
WTI Collars
Risk management derivatives
Notional Amount	381,000
Weighted-Average Floor price	68.83
Weighted-Average Ceiling price	108.61
Asset	138
WTI Collars | January 2013 to March 2013
Risk management derivatives
Notional Amount	129,000
Weighted-Average Floor price	66.24
Weighted-Average Ceiling price	105.47
Asset	13
WTI Collars | April 2013 to June 2013
Risk management derivatives
Notional Amount	126,000
Weighted-Average Floor price	70.30
Weighted-Average Ceiling price	110.43
Asset	34
WTI Collars | July 2013 to September 2013
Risk management derivatives
Notional Amount	126,000
Weighted-Average Floor price	70.00
Weighted-Average Ceiling price	110.00
Asset	$ 91