Disclosure About Oil and Gas Producing Activities (Unaudited) (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized Measure, beginning	$ 3,525,568	$ 2,026,700	$ 3,017,686
Net revisions of previous quantity estimates	(446,677)	(592,919)	(216,946)
Extensions, discoveries and other changes	1,654,793	1,601,154	782,763
Changes in future development costs	(741,658)	(606,449)	(103,056)
Sales of oil and gas, net of production costs	(896,434)	(787,409)	(513,958)
Net change in prices and production costs	108,108	1,501,002	(1,772,644)
Development costs incurred during the period that reduce future development costs	464,880	404,402	395,092
Accretion of discount	499,358	288,713	444,387
Net changes in production rates and other	(338,982)	297,957	(572,380)
Net change in income taxes	(32,900)	(607,583)	565,756
Aggregrate changes	270,488	1,498,868	(990,986)
Standardized Measure, ending	$ 3,796,056	$ 3,525,568	$ 2,026,700