Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Recognized in Other Comprehensive Income	$ (1,772)	$ 2,374	$ (1,008)	$ 1,646
Oil and gas sales [Member]
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain Reclassified into Income	$ 3,877	$ (103)	$ 5,979	$ (3)