Summary of Changes in Estimated Reserves (Unaudited) (Detail)	12 Months Ended
	Dec. 31, 2011 Mbbl		Dec. 31, 2010 Mbbl		Dec. 31, 2009 Mbbl		Dec. 31, 2008 Mbbl
Oil
Proved developed and undeveloped reserves
Proved developed and undeveloped reserves, Beginning Balance	252,114		105,349		43,164
Revisions of previous estimates	(9,278)		12,999		8,826
Acquisitions of new reserves	1,533		71,640		56,342
Extensions and discoveries	55,577		69,512		8
Sales of reserves in place	(43,331)				(97)
Production	(11,830)		(7,386)		(2,894)
Proved developed and undeveloped reserves, Ending Balance	244,785		252,114		105,349
Proved developed reserves	118,728	[1],[2]	91,965	[1]	38,327	[1]	15,342	[1]
Proved undeveloped reserves	126,057	[1],[2]	160,149	[1]	67,022	[1]	27,822	[1]
Natural Gas
Proved developed and undeveloped reserves
Proved developed and undeveloped reserves, Beginning Balance	1,762,665	[3]	680,075	[3]	1,899,636	[3]
Revisions of previous estimates	(164,845)	[3]	867,931	[3]	(1,244,873)	[3]
Acquisitions of new reserves	2,906	[3]	79,942	[3]	104,046	[3]
Extensions and discoveries	299,848	[3]	211,150	[3]	8,890	[3]
Sales of reserves in place	(476,212)	[3]	(207)	[3]	(163)	[3]
Production	(69,306)	[3]	(76,226)	[3]	(87,461)	[3]
Proved developed and undeveloped reserves, Ending Balance	1,355,056	[3]	1,762,665	[3]	680,075	[3]
Proved developed reserves	670,382	[1],[2],[3]	784,292	[1],[3]	592,777	[1],[3]	851,357	[1],[3]
Proved undeveloped reserves	684,674	[1],[2],[3]	978,373	[1],[3]	87,298	[1],[3]	1,048,279	[1],[3]

[1] Our estimated proved reserves were determined using a 12-month average price for oil and natural gas for the years ended December 31, 2011, 2010 and 2009 and year-end prices for oil and natural gas as of December 31, 2008.

[2] Proved reserves at December 31, 2011 include approximately 18,800 MBbls of oil and 45,500 MMcf of natural gas attributable to noncontrolling interests.

[3] Natural gas reserves are computed at 14.65 pounds per square inch absolute and 60 degrees Fahrenheit.