Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Recognized in Other Comprehensive Income	$ (1,617)	$ (3,510)	$ 5,120
Designated as Hedging Instrument [Member]
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Recognized in Other Comprehensive Income	(1,617)	(3,510)	5,120
Designated as Hedging Instrument [Member] | Oil and gas sales [Member]
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain Reclassified into Income	$ 994	$ 9,097	$ 2,417