Supplemental Coal Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Dec. 31, 2006
Reserve Quantities [Line Items]
Proved undeveloped reserves (Volume)	1,344,222,000	1,800,325,000	871,134,000	638,756,000
Purchases of reserves in-place	0	946,834,000	0
Sale of reserves in-place	(531,431,000)	0	0
Proved Developed and Undeveloped Reserves, Production	153,504,000	127,875,000	94,415,000
Net change due to revisions in quantity estimates	$ (83,580)	$ 414,704	$ 189,583
Steam [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserves (Volume)	3,838
Coal Segment [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserves (Volume)	4,459,000,000	4,401,000,000	4,520,000,000	4,543,000,000	4,526,000,000	4,272,000,000
Purchases of reserves in-place	6,000,000	4,000,000	5,000,000	0	177,000,000
Sale of reserves in-place	0	(41,000,000)	(3,000,000)	(12,000,000)	(33,000,000)
Proved Developed and Undeveloped Reserves, Production	(63,000,000)	(63,000,000)	(59,000,000)	(65,000,000)	(65,000,000)
Net change due to revisions in quantity estimates	$ 115,000	$ (19,000)	$ 34,000	$ 94,000	$ 175,000
Proportionate share of proved and probable reserves of unconsolidated equity affiliates.........................................................................................	145,000,000	172,000,000	170,000,000	171,000,000	179,000,000
metallurgical coal [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserves (Volume)	621,000,000