Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Coal and other plant and equipment	$ 3,731,618	$ 3,681,051
Drilling Cost	2,388,790	1,937,336
Proven gas properties	1,684,675	1,670,404
Coal properties and surface lands	1,510,307	1,463,406
Unproven gas properties	1,411,574	1,409,408
Gas gathering equipment	1,082,355	1,058,008
Airshafts	850,771	688,548
Mine development	453,689	397,466
Leased coal lands	416,733	354,607
Gas wells and related equipment	397,015	381,348
Coal advance mining royalties	388,033	388,020
Other gas assets	125,484	126,239
Gas advance royalties	22,284	22,668
Total property plant and equipment	14,463,328	13,578,509
Less Accumulated depreciation, depletion and amortization	4,499,344	4,136,247
Total Property, Plant and Equipment-Net	$ 9,963,984	$ 9,442,262