Impact of Derivative Instruments on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Commodity Price Risk Management (loss), net
Derivative [Line Items]
Net settlements	$ 66,831	$ 50,412
Net change in fair value of unsettled derivatives	(55,775)	16,250
Total commodity price risk management gain (loss), net	11,056	66,662
Sales From Natural Gas Marketing
Derivative [Line Items]
Net settlements	245	232
Net change in fair value of unsettled derivatives	(220)	(170)
Total commodity price risk management gain (loss), net	25	62
Cost of Natural Gas Marketing
Derivative [Line Items]
Net settlements	(228)	(218)
Net change in fair value of unsettled derivatives	225	150
Total commodity price risk management gain (loss), net	$ (3)	$ (68)