Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on the Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative instruments and hedging activities
Gain (loss) on derivative instruments	$ 17,364	$ 71,906	$ 44,668	$ 69,577
Unrealized gain (loss) on derivative instruments			44,668	69,577
Gain (loss) on derivative instruments	17,364	50,349	44,668	25,064
Total	17,364	50,349	44,668	(45,711)
Derivatives Designated as Hedges
Derivative instruments and hedging activities
Gain (loss) on derivative instruments	0	0	0	(70,775)
Derivatives Designated as Hedges | Natural gas revenues
Derivative instruments and hedging activities
Gain (loss) on derivative instruments	0	0	0	(70,557)
Derivatives Designated as Hedges | Crude oil and condensate revenues
Derivative instruments and hedging activities
Gain (loss) on derivative instruments	0	0	0	(218)
Derivatives Not Designated as Hedges
Derivative instruments and hedging activities
Unrealized gain (loss) on derivative instruments	(27,733)	31,833	(89,159)	44,766
Derivatives Not Designated as Hedges | Natural gas revenues
Derivative instruments and hedging activities
Gain (loss) on derivative instruments	0	(21,427)	0	(43,747)
Derivatives Not Designated as Hedges | Crude oil and condensate revenues
Derivative instruments and hedging activities
Gain (loss) on derivative instruments	0	(130)	0	(766)
Derivatives Not Designated as Hedges | Gain (loss) on derivative instruments
Derivative instruments and hedging activities
Gain (loss) on derivative instruments	$ 45,097	$ 40,073	$ 133,827	$ 24,811