Derivative Instruments--Effect of Derivative Instruments on the Consolidated Statements of Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Non-Hedge Derivative Gains (Losses) [Line Items]
Amortization of premium	$ (166)	$ 0
Non-hedge derivative losses	(29,199)	(15,546)
Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(5,193)	(19,364)
(Payments on) receipts from settlement of non-hedge derivative instruments	(5,850)	4,525
Costless Collars
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(2,863)	(8,285)
(Payments on) receipts from settlement of non-hedge derivative instruments	(41)	5,731
Enhanced Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
(Payments on) receipts from settlement of non-hedge derivative instruments	(1,992)	0
Enhanced Swaps and Put Options
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(10,361)	2,031
Basis Protection Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(1,861)	176
(Payments on) receipts from settlement of non-hedge derivative instruments	$ (872)	$ (360)