Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Coal and other plant and equipment	$ 6,032,128	$ 6,030,620
Drilling Cost	1,623,391	1,550,297
Proven gas properties	1,600,532	1,596,838
Coal properties and surface lands	1,351,271	1,346,151
Unproven gas properties	1,321,108	1,266,017
Gas gathering equipment	1,024,952	1,006,882
Airshafts	711,586	706,388
Mine development	561,288	537,939
Leased coal lands	529,741	529,758
Gas wells and related equipment	508,768	492,367
Coal advance mining royalties	394,438	391,501
Other gas assets	82,094	82,217
Gas advance royalties	8,226	8,229
Total property plant and equipment	15,749,523	15,545,204
Less Accumulated depreciation, depletion and amortization	5,516,319	5,354,237
Total Property, Plant and Equipment-Net	$ 10,233,204	$ 10,190,967