Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 1,025,044	$ 1,590,625	$ 1,405,262
Crude oil and condensate revenues	248,211	313,889	291,418
Income (loss) before income taxes	(187,273)	32,401	485,538
Tax expense	73,382	72,067	(205,765)
Net income (loss)	(113,891)	104,468	279,773
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	0	(144,177)	56,361
Tax expense	0	57,467	(22,199)
Net income (loss)	0	(86,710)	34,162
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	(143,577)	52,733
Crude oil and condensate revenues	0	(626)	4,269
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
Amortization of net (gain) loss	$ 0	$ 26	$ (641)