Supplemental Coal Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 T	Dec. 31, 2013 T	Dec. 31, 2012 T	Dec. 31, 2011 T	Dec. 31, 2010 T	Dec. 31, 2009 T
Reserve Quantities [Line Items]
Proven properties		$ 1,670,404
Proved Oil and Gas Property, Successful Effort Method	1,768,007	1,670,404
Reserve Quantities [Roll Forward]
Proved Developed and Undeveloped Reserves, Production	235,714,000	172,380,000	156,325,000
Net change due to revisions in quantity estimates	151,233	132,900	(104,158)
Unproven properties		1,463,406
Unproved Oil and Gas Property, Successful Effort Method	1,540,835	1,463,406
Steam [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	2,759,000
Coal Segment [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	3,238,000,000	3,032,000,000	4,229,000,000	4,314,000,000	4,229,000,000	4,350,000,000
Reserve Quantities [Roll Forward]
Purchase of minerals in place (Mass)	0	1,000,000	0	6,000,000	4,000,000
Sales of minerals in place (Mass)	(233,000,000)	(1,199,000,000)	(155,000,000)	0	(41,000,000)
Production of proved reserves (Mass)	(32,000,000)	(55,000,000)	(55,000,000)	(62,000,000)	(62,000,000)
Net change due to revisions in quantity estimates	$ 471,000	$ 56,000	$ 125,000	$ 141,000	$ (22,000)
metallurgical coal [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	480,000,000
Affiliated Entity [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)		55,000,000	57,000,000	41,000,000	145,000,000	172,000,000