Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Coal and other plant and equipment	$ 3,711,184	$ 3,681,051
Drilling Cost	2,228,806	1,937,336
Proven gas properties	1,683,052	1,670,404
Coal properties and surface lands	1,493,708	1,463,406
Unproven gas properties	1,407,518	1,409,408
Gas gathering equipment	1,074,265	1,058,008
Airshafts	780,212	688,548
Mine development	443,657	397,466
Leased coal lands	416,296	354,607
Gas wells and related equipment	388,033	388,020
Coal advance mining royalties	387,199	381,348
Other gas assets	124,903	126,239
Gas advance royalties	22,134	22,668
Total property plant and equipment	14,160,967	13,578,509
Less Accumulated depreciation, depletion and amortization	4,384,209	4,136,247
Total Property, Plant and Equipment-Net	$ 9,776,758	$ 9,442,262