Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2010
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain (Loss) Recognized in Other Comprehensive Income	$ (3,510)	$ 5,120	$ (3,510)	$ 5,120	$ (2,857)
Oil and gas sales [Member]
Effect of Derivative Instruments on the Consolidated Statement of Operations
Amount of Gain Reclassified into Income	$ 9,097	$ 2,417			$ 17,538