Derivative Instruments and Hedging Activities (Details 4) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative instruments and hedging activities
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into income (effective portion)	$ 0	$ (22,956,000)	$ 0	$ (93,731,000)
Unrealized gain (loss) on derivative instruments			27,304,000	(2,329,000)
Gain (loss) on derivative instruments	(6,819,000)	(25,285,000)	27,304,000	(25,285,000)
Total	(6,819,000)	(25,285,000)	27,304,000	(96,060,000)
Ineffectiveness related to derivative instruments		0		0
Natural Gas Revenues
Derivative instruments and hedging activities
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into income (effective portion)	0	(22,320,000)	0	(92,877,000)
Crude Oil and Condensate Revenues
Derivative instruments and hedging activities
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into income (effective portion)	0	(636,000)	0	(854,000)
Commodity contracts
Derivative instruments and hedging activities
Amount of gain (loss) recognized in other comprehensive income (loss) on derivative (effective portion)	0	0	0	(133,310,000)
Derivatives Designated as Hedges
Derivative instruments and hedging activities
Realized gain (loss) on derivative instruments	0	0	0	(70,775,000)
Derivatives Designated as Hedges | Natural Gas Revenues
Derivative instruments and hedging activities
Realized gain (loss) on derivative instruments	0	0	0	(70,557,000)
Derivatives Designated as Hedges | Crude Oil and Condensate Revenues
Derivative instruments and hedging activities
Realized gain (loss) on derivative instruments	0	0	0	(218,000)
Derivatives Not Designated as Hedges
Derivative instruments and hedging activities
Unrealized gain (loss) on derivative instruments	(57,864,000)	12,933,000	(61,426,000)	12,933,000
Derivatives Not Designated as Hedges | Natural Gas Revenues
Derivative instruments and hedging activities
Realized gain (loss) on derivative instruments	0	(22,320,000)	0	(22,320,000)
Derivatives Not Designated as Hedges | Crude Oil and Condensate Revenues
Derivative instruments and hedging activities
Realized gain (loss) on derivative instruments	0	(636,000)	0	(636,000)
Derivatives Not Designated as Hedges | Gain (loss) on derivative instruments
Derivative instruments and hedging activities
Realized gain (loss) on derivative instruments	$ 51,045,000	$ (15,262,000)	$ 88,730,000	$ (15,262,000)