Oil and Gas Reserve Information Unaudited (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Standardized measure of future net cash flows related to estimated proved oil and gas reserves
Future cash inflows	$ 6,171,279	$ 3,803,004	$ 2,663,285
Future production costs	(1,747,806)	(1,191,718)	(950,434)
Future development costs	(1,219,214)	(907,956)	(912,500)
Future income taxes	(852,364)	(330,651)	(38,845)
Future net cash flows	2,351,895	1,372,679	761,506
10% annual discount	(808,933)	(415,050)	(146,519)
Standardized measure of discounted future net cash flows	1,542,962	957,629	614,987
Changes in standardized measure of future net cash flows related to estimated proved oil and gas reserves
Standardized measure at beginning of year	957,629	614,987	793,104
Sales and transfers of oil and gas produced, net of production costs	(670,347)	(487,418)	(546,737)
Changes in price, net of future production costs	502,324	485,272	284,504
Extensions and discoveries, net of future production and development costs	293,168	270,629	21,249
Changes in estimated future development costs, net of development costs incurred during the period	97,852	119,986	183,058
Revisions of quantity estimates	(27,854)	147,509	(150,609)
Accretion of discount	118,722	64,836	79,904
Net change in income taxes	(300,363)	(196,219)	(27,436)
Purchases of reserves in-place	567,286	21,264
Sales of reserves in-place	(36,278)	1,424	3,152
Changes in production rates due to timing and other	40,823	(84,641)	(25,202)
Net increase (decrease) in standardized measure	585,333	342,642	(178,117)
Standardized measure at end of year	$ 1,542,962	$ 957,629	$ 614,987