Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid		$ 66,870
Net derivative asset position, Fair value of derivatives		(20,480)
Net derivative asset position, net		46,390
Net derivative liability position, Collateral paid	7,289		46,305
Net derivative liability position, Fair value of derivatives	(19,578)		(87,125)
Net derivative liability position, net	$ (12,289)		$ (40,820)