Property, Plant and Equipment PROPERTY PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Impairment of Oil and Gas Properties	$ 0	$ 828,905	$ 0
Proved Oil and Gas Property, Successful Effort Method	2,016,916	1,922,602
Drilling Cost	3,583,565	3,452,989
Unproved Oil and Gas Property, Successful Effort Method	1,116,282	1,421,083
Property, Plant and Equipment, Gross	13,771,388	13,794,907
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	5,630,949	5,062,201
Total Property, Plant and Equipment-Net	8,140,439	9,669,377
Property, Plant, and Equipment, Owned, Net	8,140,439	8,732,706
Gas Segment [Member]
Proved Oil and Gas Property, Successful Effort Method		1,922,602
Drilling Cost	3,583,565	3,452,989
Unproved Oil and Gas Property, Successful Effort Method		1,421,083
Gas Gathering Systems Net	1,138,299	1,147,173
Gas wells and related equipment	791,996	785,744
Other gas assets	190,406	125,691
Advances for Gas Exploration, Development and Production Amount	13,762	19,745
Property, Plant and Equipment, Gross	8,851,226	8,875,027
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	3,106,296	2,695,674
Total Property, Plant and Equipment-Net	5,744,930	6,179,353
PA Operations [Member]
Coal and other plant and equipment	2,307,668	2,284,103
Coal Properties And Surface Lands	458,398	456,044
Property, Plant and Equipment, Other, Gross	371,752	351,870
Mine development	326,152	326,225
Prepaid Royalties	16,224	16,263
Property Subject to or Available for Operating Lease, Gross	26,566	26,402
Property, Plant and Equipment, Gross	3,506,760	3,460,907
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	1,768,712	1,603,642
Total Property, Plant and Equipment-Net	1,738,048	1,857,265
Other Segments [Member]
Coal and other plant and equipment	532,919	569,333
Coal Properties And Surface Lands	481,126	484,348
Property, Plant and Equipment, Other, Gross	10,003	10,002
Mine development	17,988	18,073
Prepaid Royalties	310,530	312,452
Property Subject to or Available for Operating Lease, Gross	60,836	64,765
Property, Plant and Equipment, Gross	1,413,402	1,458,973
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	755,941	762,885
Total Property, Plant and Equipment-Net	657,461	$ 696,088
Proved Developed [Member]
Impairment of Oil and Gas Properties	824,742
proved undeveloped [Member]
Impairment of Oil and Gas Properties	$ 4,163