Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Loss Recognized in Other Comprehensive Income	$ (3,418)	$ (3,910)
Location of Gain (Loss) Reclassified into Income	Oil and gas sales	Oil and gas sales
Designated as Hedging Instrument [Member]
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Loss Recognized in Other Comprehensive Income	(6,821)	(3,523)
Designated as Hedging Instrument [Member] | Oil and gas sales [Member]
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Reclassified into Income	$ (3,403)	$ 387