Supplementary Oil and Natural Gas Disclosures (Unaudited) (Schedule of Estimated Net Proved Reserves, Changes in Estimated Net Proved Reserves and Proved Developed Reserves) (Detail)	12 Months Ended
	Dec. 31, 2012 MMBbls	Dec. 31, 2011 MMBbls	Dec. 31, 2010 MMBbls
Crude Oil (Mbbls) [Member]
Estimated Proved Reserves, Beginning Balance	27,301	17,223	19,923
Acquisitions	16,430	7,987
Extensions, discoveries and other additions	6,388	2,266	652
Revisions	1,128	2,778	(1,016)
Production	(3,805)	(2,953)	(2,336)
Estimated Proved Reserves, Ending Balance	47,442	27,301	17,223
Proved Developed Reserves (Volume)	37,908	24,791	15,974
Gas (Mmcf) [Member]
Estimated Proved Reserves, Beginning Balance	58,785	61,251	67,378
Acquisitions	115,876	8,640
Extensions, discoveries and other additions	10,241	4,664	489
Revisions	4,033	(6,678)	8,892
Production	(8,996)	(9,092)	(15,508)
Estimated Proved Reserves, Ending Balance	179,939	58,785	61,251
Proved Developed Reserves (Volume)	120,687	52,739	56,410
Oil Equivalent (Mboe) [Member]
Estimated Proved Reserves, Beginning Balance	37,099	27,431	31,153
Acquisitions	35,742	9,427
Extensions, discoveries and other additions	8,095	3,043	733
Revisions	1,800	1,666	466
Production	(5,304)	(4,468)	(4,921)
Estimated Proved Reserves, Ending Balance	77,432	37,099	27,431
Proved Developed Reserves (Volume)	58,022	33,581	25,376