Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative [Line Items]
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into income (effective portion)	$ 0	$ (21,557)	$ 0	$ (115,288)
Natural gas revenues
Derivative [Line Items]
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into income (effective portion)	0	(21,427)	0	(114,304)
Crude oil and condensate revenues
Derivative [Line Items]
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into income (effective portion)	0	(130)	0	(984)
Commodity contracts
Derivative [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss) on derivative (effective portion)	$ 0	$ 0	$ 0	$ (133,310)