Oil and Gas Reserve Information Unaudited (Details)	12 Months Ended
	Dec. 31, 2011 MMBbls	Dec. 31, 2010 MMBbls	Dec. 31, 2009 MMBbls	Dec. 31, 2008 MMBbls
Oil (MBbls) [Member]
Estimated quantities of net proved and proved developed oil and natural gas reserves
Estimated proved reserves, beginning balance	45,754	33,203	32,336	36,564
Revisions of previous estimates	2,988	3,299	1,964
Extensions, discoveries and other additions	3,544	2,668	417
Purchase of producing properties	14,396	637
Sale of reserves	(1,950)	(23)	(402)
Production	(6,427)	(5,714)	(6,207)
Estimated proved developed reserves
Proved developed reserves, beginning balance	31,026	25,000	24,380
Natural Gas (MMcf) [Member]
Estimated quantities of net proved and proved developed oil and natural gas reserves
Estimated proved reserves, beginning balance	327,352	274,705	216,694	299,554
Revisions of previous estimates	(23,801)	13,439	(53,423)
Extensions, discoveries and other additions	93,520	82,846	12,198
Purchase of producing properties	24,595	3,816
Sale of reserves	(2,150)	(153)	(300)
Production	(39,517)	(41,937)	(41,335)
Estimated proved developed reserves
Proved developed reserves, beginning balance	174,067	174,876	172,452
Oil And Natural Gas (MMcfe) [Member]
Estimated quantities of net proved and proved developed oil and natural gas reserves
Estimated proved reserves, beginning balance	601,880	473,923	410,711	518,935
Revisions of previous estimates	(5,869)	33,231	(41,636)
Extensions, discoveries and other additions	114,784	98,854	14,703
Purchase of producing properties	110,971	7,637
Sale of reserves	(13,850)	(289)	(2,714)
Production	(78,079)	(76,221)	(78,577)
Estimated proved developed reserves
Proved developed reserves, beginning balance	360,224	324,876	318,729