Supplemental Coal Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 T	Dec. 31, 2012 T	Dec. 31, 2011 T	Dec. 31, 2010 T	Dec. 31, 2009 T	Dec. 31, 2008 T
Reserve Quantities [Roll Forward]
Sale of reserves in-place	0	0	(531,431,000)
Proved Developed and Undeveloped Reserves, Production	172,380,000	156,325,000	153,504,000
Net change due to revisions in quantity estimates	$ 132,900	$ (104,158)	$ (83,580)
Steam [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	2,511,000
Coal Segment [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	3,032,000,000	4,229,000,000	4,314,000,000	4,229,000,000	4,350,000,000	4,372,000,000
Reserve Quantities [Roll Forward]
Purchase of minerals in place (Mass)	1,000,000	0	6,000,000	4,000,000	5,000,000
Sales of minerals in place (Mass)	(1,199,000,000)	(155,000,000)	0	(41,000,000)	(3,000,000)
Production of proved reserves (Mass)	(55,000,000)	(55,000,000)	(62,000,000)	(62,000,000)	(58,000,000)
Net change due to revisions in quantity estimates	$ 56,000	$ 125,000	$ 141,000	$ (22,000)	$ 34,000
metallurgical coal [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	521,000,000
Affiliated Entity [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)		57,000,000	41,000,000	145,000,000	172,000,000	170,000,000