Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Recognized in Other Comprehensive Income	$ 6,516	$ (1,617)	$ (3,510)
Designated as Hedging Instrument
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Recognized in Other Comprehensive Income	6,516	(1,617)	(3,510)
Designated as Hedging Instrument | Oil and gas sales
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Reclassified into Income	$ (3,044)	$ 927	$ 9,097