Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 224,806	$ 437,761	$ 584,997	$ 870,571
Crude oil and condensate revenues	81,233	86,341	143,791	145,485
Total before tax	(43,130)	194,319	20,599	372,249
Tax benefit (expense)	15,622	(75,899)	(7,852)	(146,798)
NET INCOME (LOSS)	(27,508)	118,420	12,747	225,451
Amount reclassified from accumulated other comprehensive income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Total before tax	0	(22,956)	0	(93,731)
Tax benefit (expense)	0	9,149	0	37,359
NET INCOME (LOSS)	0	(13,807)	0	(56,372)
Gain (loss) 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	(22,320)	0	(92,877)
Crude oil and condensate revenues	$ 0	$ (636)	$ 0	$ (854)