Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Estimated fair value of Company's commodity derivative instruments for cash collateral and associated cash as collateral
Net derivative asset position, Collateral paid	$ 88,246	$ 66,870	$ (60,247)
Net derivative asset position, Fair value of derivatives	(73,424)	(20,480)	139,882
Net derivative asset position, net	14,822	46,390	79,635
Net derivative liability position, Collateral paid	0	0	0
Net derivative liability position, Fair value of derivatives	0	0	0
Net derivative liability position, net	$ 0	$ 0	$ 0