Income Taxes - Net Deferred Tax Assets/Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets:
Net Operating Loss- Federal	$ 215,936	$ 202,913
Net Operating Loss - State	129,641	130,430
Foreign Tax Credit	43,194	43,194
Operating Lease Right-of-Use Assets	28,085	47,849
Gas Well Closing	24,251	17,888
Salary Retirement	11,478	9,236
Equity Compensation	6,639	9,308
Alternative Minimum Tax	0	51,241
Interest Limitation	0	25,734
Other	9,416	10,030
Total Deferred Tax Assets	468,640	547,823
Valuation Allowance	(123,098)	(125,054)
Net Deferred Tax Assets	345,542	422,769
Deferred Tax Liabilities:
Property, Plant and Equipment	(649,917)	(593,401)
Investment in Partnership	(85,882)	(145,424)
Gas Derivatives	(26,882)	(105,721)
Operating Lease Liabilities	(28,287)	(46,640)
Discount on Convertible Notes	(18,097)	0
Advance Gas Royalties	(2,519)	(3,337)
Other	(211)	(4,354)
Total Deferred Tax Liabilities	(811,795)	(898,877)
Net Deferred Tax Liability	$ (466,253)	$ (476,108)