Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 1,022,590	$ 1,025,044	$ 1,590,625
Crude oil and condensate revenues	151,106	248,211	313,889
General and administrative	87,242	69,444	82,590
Income (loss) before income taxes	(659,599)	(187,273)	32,401
Income tax benefit	242,475	73,382	72,067
NET INCOME (LOSS)	(417,124)	(113,891)	104,468
Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Income (loss) before income taxes	(111)	0	(144,177)
Income tax benefit	41	0	57,467
NET INCOME (LOSS)	(70)	0	(86,710)
Net gains (losses) on cash flow hedges | Amount Reclassified from Accumulated Other Comprehensive Income | Commodity contracts
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	0	0	(143,577)
Crude oil and condensate revenues	0	0	(626)
Amortization of prior service cost | Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative	111	0	0
Amortization of net (gain) loss | Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative	$ 0	$ 0	$ (26)