Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Coal and other plant and equipment	$ 6,207,105	$ 6,030,620
Drilling Cost	1,830,666	1,550,297
Proven gas properties	1,601,106	1,596,838
Coal properties and surface lands	1,449,526	1,346,151
Unproven gas properties	1,383,921	1,266,017
Gas gathering equipment	1,046,495	1,006,882
Airshafts	746,134	706,388
Mine development	608,630	537,939
Leased coal lands	529,409	529,758
Gas wells and related equipment	623,176	492,367
Coal advance mining royalties	397,015	391,501
Other gas assets	125,635	82,217
Gas advance royalties	22,286	8,229
Total property plant and equipment	16,571,104	15,545,204
Less Accumulated depreciation, depletion and amortization	5,940,247	5,354,237
Total Property, Plant and Equipment-Net	$ 10,630,857	$ 10,190,967