Impact of Derivative Instruments on Statement of Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Commodity Price Risk Management (loss), net
Derivative [Line Items]
Realized gains	$ (1,525)	$ 13,095	$ 10,847	$ 39,200
Unrealized gains (Losses)	(22,113)	(45,038)	(32,116)	(20,913)
Total derivatives gain (loss)	(23,638)	(31,943)	(21,269)	18,287
Sales From Natural Gas Marketing
Derivative [Line Items]
Realized gains	240	401	267	1,946
Unrealized gains (Losses)	(311)	(790)	340	(1,786)
Total derivatives gain (loss)	(71)	(389)	607	160
Cost of Natural Gas Marketing
Derivative [Line Items]
Realized gains	(188)	(384)	(125)	(1,847)
Unrealized gains (Losses)	278	831	(281)	1,782
Total derivatives gain (loss)	90	447	(406)	(65)
Reclassification of Realized Gains (Losses) Included in Prior Periods Unrealized | Commodity Price Risk Management (loss), net
Derivative [Line Items]
Realized gains	2,936	15,010	25,273	22,813
Unrealized gains (Losses)	(2,936)	(15,010)	(25,273)	(22,813)
Total derivatives gain (loss)	0	0	0	0
Reclassification of Realized Gains (Losses) Included in Prior Periods Unrealized | Sales From Natural Gas Marketing
Derivative [Line Items]
Realized gains	249	386	206	1,358
Unrealized gains (Losses)	(249)	(386)	(206)	(1,358)
Total derivatives gain (loss)	0	0	0	0
Reclassification of Realized Gains (Losses) Included in Prior Periods Unrealized | Cost of Natural Gas Marketing
Derivative [Line Items]
Realized gains	(196)	(364)	(133)	(1,195)
Unrealized gains (Losses)	196	364	133	1,195
Total derivatives gain (loss)	0	0	0	0
Realized and Unrealized Gains (Losses) for the Current Period | Commodity Price Risk Management (loss), net
Derivative [Line Items]
Realized gains	(4,461)	(1,915)	(14,426)	16,387
Unrealized gains (Losses)	(19,177)	(30,028)	(6,843)	1,900
Total derivatives gain (loss)	(23,638)	(31,943)	(21,269)	18,287
Realized and Unrealized Gains (Losses) for the Current Period | Sales From Natural Gas Marketing
Derivative [Line Items]
Realized gains	(9)	15	61	588
Unrealized gains (Losses)	(62)	(404)	546	(428)
Total derivatives gain (loss)	(71)	(389)	607	160
Realized and Unrealized Gains (Losses) for the Current Period | Cost of Natural Gas Marketing
Derivative [Line Items]
Realized gains	8	(20)	8	(652)
Unrealized gains (Losses)	82	467	(414)	587
Total derivatives gain (loss)	$ 90	$ 447	$ (406)	$ (65)