Derivatives (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended																						3 Months Ended																			3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 International Coal Forward Contracts	Dec. 31, 2012 International Coal Forward Contracts	Mar. 31, 2013 International Coal Forward Contracts 2013	Dec. 31, 2012 International Coal Forward Contracts 2013	Mar. 31, 2013 International Coal Forward Contracts 2014	Dec. 31, 2012 International Coal Forward Contracts 2014	Mar. 31, 2013 International Coal Forward Contracts 2015	Dec. 31, 2012 International Coal Forward Contracts 2015	Mar. 31, 2013 International Coal Forward Contracts 2016	Dec. 31, 2012 International Coal Forward Contracts 2016	Mar. 31, 2013 International Coal Forward Contracts Underlying basis Tons T	Dec. 31, 2012 International Coal Forward Contracts Underlying basis Tons T	Mar. 31, 2013 International Coal Forward Contracts Underlying basis Tons 2013 T	Dec. 31, 2012 International Coal Forward Contracts Underlying basis Tons 2013 T	Mar. 31, 2013 International Coal Forward Contracts Underlying basis Tons 2014 T	Dec. 31, 2012 International Coal Forward Contracts Underlying basis Tons 2014 T	Mar. 31, 2013 International Coal Forward Contracts Underlying basis Tons 2015 T	Dec. 31, 2012 International Coal Forward Contracts Underlying basis Tons 2015 T	Mar. 31, 2013 International Coal Forward Contracts Underlying basis Tons 2016 T	Dec. 31, 2012 International Coal Forward Contracts Underlying basis Tons 2016 T	Jun. 30, 2012 WTI Collars	Mar. 31, 2013 WTI Collars	Dec. 31, 2012 WTI Collars	Dec. 31, 2012 WTI Collars January 2013 to March 2013	Mar. 31, 2013 WTI Collars April 2013 to June 2013	Dec. 31, 2012 WTI Collars April 2013 to June 2013	Mar. 31, 2013 WTI Collars July 2013 to September 2013	Dec. 31, 2012 WTI Collars July 2013 to September 2013	Mar. 31, 2013 WTI Collars October 2013 to December 2013	Mar. 31, 2013 WTI Collars January 2014 to March 2014	Mar. 31, 2013 WTI Collars Underlying basis Barrels bbl	Dec. 31, 2012 WTI Collars Underlying basis Barrels bbl	Dec. 31, 2012 WTI Collars Underlying basis Barrels January 2013 to March 2013 bbl	Mar. 31, 2013 WTI Collars Underlying basis Barrels April 2013 to June 2013 bbl	Dec. 31, 2012 WTI Collars Underlying basis Barrels April 2013 to June 2013 bbl	Mar. 31, 2013 WTI Collars Underlying basis Barrels July 2013 to September 2013 bbl	Dec. 31, 2012 WTI Collars Underlying basis Barrels July 2013 to September 2013 bbl	Mar. 31, 2013 WTI Collars Underlying basis Barrels October 2013 to December 2013 bbl	Mar. 31, 2013 WTI Collars Underlying basis Barrels January 2014 to March 2014 bbl	Mar. 31, 2013 Domestic Coal Futures Contracts Underlying basis Tons T	Mar. 31, 2013 Domestic Coal Futures Contracts Underlying basis Tons 2014 T
Risk management derivatives
Notional Amount													2,246,000	1,058,000	646,000	516,000	1,124,000	198,000	344,000	212,000	132,000	132,000											534,000	381,000	129,000	126,000	126,000	126,000	126,000	138,000	144,000	420,000	420,000
Weighted-Average Floor price																																	71.86	68.83	66.24	70.30	70.30	70.00	70.00	73.78	73.00
Weighted-Average Ceiling price																																	112.47	108.61	105.47	110.43	110.43	110.00	110.00	113.85	115.10
Gross Amounts of Recognized Assets			$ 28,278	$ 13,677	$ 12,964	$ 9,288	$ 9,831	$ 2,776	$ 4,268	$ 1,598	$ 1,215	$ 15												$ 60	$ 138	$ 13	$ 5	$ 34	$ 41	$ 91	$ 14
Gross Amounts offset in the Consolidated Balance Sheets			(848)	(30)	(848)							(30)												(29)								(29)
Amounts of Assets Presented in the Consolidated Balance Sheets			27,430	13,647	12,116	9,288	9,831	2,776	4,268	1,598	1,215	(15)												31	138	13	5	34	41	91	14	(29)
Gain																																										67	67
Loss																																										(91)	(91)
Amount paid or received																							0
Total Derivative Activity
Derivative financial instruments net asset at the beginning of the period	13,785	2,275
Total derivative mark-to-market gains (losses)	13,652	(2,056)
Total derivative cash settlements paid (received)	24	(524)
Derivative financial instruments net asset (liability) at the end of the period	$ 27,461	$ (305)