Supplemental Gas Data PV-10 Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Net changes in sales prices and production costs	$ (339,098)	$ 721,997	$ (457,138)
Net changes in sales prices and production costs	(217,186)	(286,883)	(335,706)
Net change due to revisions in quantity estimates	(83,580)	414,704	189,583
Net change due to extensions, discoveries and improved recovery	324,755	326,584	124,008
Net change due to (divestiture) acquisition	(559,132)	500,376	0
Development costs incurred during the period	463,401	295,142	181,944
Difference in previously estimated development costs compared to actual costs incurred during the period	154,137	(12,060)	(3,282)
Changes in estimated future development costs	155,619	(426,870)	(380,639)
Net change in future income taxes	130,746	(612,114)	248,639
Accretion of discount and other	56,698	(154,406)	108,508
Total discounted cash flow at end of period	$ 1,747,181	$ 1,660,821	$ 894,351	$ 1,218,434