Impact of Derivative Instruments on 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
Commodity Price Risk Management (loss), net
Derivative [Line Items]
Net settlements	$ 67,993	$ (4,459)	$ 162,454	$ (21,511)
Net change in fair value of unsettled derivatives	55,556	94,672	(21,284)	34,172
Total commodity price risk management gain (loss), net	123,549	90,213	141,170	12,661
Sales From Natural Gas Marketing
Derivative [Line Items]
Net settlements	165	210	561	(376)
Net change in fair value of unsettled derivatives	(5)	170	(298)	123
Total commodity price risk management gain (loss), net	160	380	263	(253)
Cost of Natural Gas Marketing
Derivative [Line Items]
Net settlements	(157)	(182)	(531)	502
Net change in fair value of unsettled derivatives	(5)	(191)	260	(199)
Total commodity price risk management gain (loss), net	$ (162)	$ (373)	$ (271)	$ 303