Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 222,963	$ 347,970	$ 807,960	$ 1,218,540
Crude oil and condensate revenues	59,014	82,563	202,804	228,047
Total before tax	(25,534)	172,919	(4,936)	545,167
Tax benefit (expense)	10,020	(72,131)	2,169	(218,928)
NET INCOME (LOSS)	(15,514)	100,788	(2,767)	326,239
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	(21,557)	0	(115,288)
Tax benefit (expense)	0	8,592	0	45,951
NET INCOME (LOSS)	0	(12,965)	0	(69,337)
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	(21,427)	0	(114,304)
Crude oil and condensate revenues	$ 0	$ (130)	$ 0	$ (984)