Derivatives (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Amount of Gain or (Loss) Recognized in Earnings
Realized	$ (3,410)	$ 42,521	$ 9,875
Unrealized	17,858	4,506	(9,153)
Total	14,448	47,027	722
Commodity Derivatives | Nondesignated
Amount of Gain or (Loss) Recognized in Earnings
Realized	(3,410)	42,521	9,875
Unrealized	17,858	4,506	(9,153)
Total	$ 14,448	$ 47,027	$ 722