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Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Property, Plant and Equipment [Line Items]      
Impairment of Oil and Gas Properties $ 137,865 $ 0  
Unproven gas properties 1,045,437   $ 1,116,282
Drilling Cost 3,465,645   3,583,565
Gas gathering equipment 1,127,614   1,138,299
Gas wells and related equipment 763,248   791,996
Other gas assets 189,481   190,406
Gas advance royalties 13,186   13,762
Proven gas properties 1,982,134   2,016,916
Coal and other plant and equipment 2,318,308   2,307,668
Coal Lands and or Surface Rights 458,976   458,398
Property, Plant and Equipment, Other, Gross 372,656   371,752
Mine development 326,152   326,152
Coal advance mining royalties 16,108   16,224
Leased coal lands 26,556   26,566
Property, Plant and Equipment 13,518,261   13,771,388
Less Accumulated depreciation, depletion and amortization 5,695,342   5,630,949
Total Property, Plant and Equipment-Net 7,986,541   8,140,439
Assets Held-for-sale, Long Lived, Fair Value Disclosure 163,622   0
Property, Plant, and Equipment, Owned, Net 7,986,541   8,140,439
Gas Segment [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment 8,586,745   8,851,226
Less Accumulated depreciation, depletion and amortization 3,126,778   3,106,296
Total Property, Plant and Equipment-Net 5,459,967   5,744,930
PA Operations [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment 3,518,756   3,506,760
Less Accumulated depreciation, depletion and amortization 1,810,108   1,768,712
Total Property, Plant and Equipment-Net 1,708,648   1,738,048
Other Segments [Member]      
Property, Plant and Equipment [Line Items]      
Coal and other plant and equipment 531,692   532,919
Coal Lands and or Surface Rights 481,037   481,126
Property, Plant and Equipment, Other, Gross 10,002   10,003
Mine development 17,988   17,988
Coal advance mining royalties 311,205   310,530
Leased coal lands 60,836   60,836
Property, Plant and Equipment 1,412,760   1,413,402
Less Accumulated depreciation, depletion and amortization 758,456   755,941
Total Property, Plant and Equipment-Net $ 654,304   $ 657,461