Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Property, Plant and Equipment [Line Items]
Coal and other plant and equipment	$ 5,160,759	$ 5,100,085
Proven gas properties	1,542,837	1,662,605
Coal Properties And Surface Lands	1,340,757	1,292,701
Intangible drilling cost	1,277,678	1,116,884
Unproved Oil and Gas Property, Successful Effort Method	1,258,027	2,206,399
Gas gathering equipment	963,494	941,772
Airshafts	659,736	662,315
Leased coal lands	540,817	536,603
Mine development	457,179	587,518
Gas wells and related equipment	408,814	367,448
Coal advance mining royalties	393,340	389,379
Other gas assets	79,816	84,571
Gas advance royalties	4,065	3,078
Total property plant and equipment	14,087,319	14,951,358
Less Accumulated depreciation, depletion and amortization	4,760,903	4,822,107
Total Property, Plant and Equipment-Net	$ 9,326,416	$ 10,129,251	$ 6,124,290