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, 2013 MMBbls	Dec. 31, 2012 MMBbls	Dec. 31, 2011 MMBbls
Crude Oil (Mbbls) [Member]
Estimated Proved Reserves, Beginning Balance	47,442	27,301	17,223
Acquisitions	366	16,430	7,987
Sales	(1,415)
Extensions, discoveries and other additions	7,354	6,388	2,266
Revisions	3,952	1,128	2,778
Production	(6,182)	(3,805)	(2,953)
Estimated Proved Reserves, Ending Balance	51,517	47,442	27,301
Proved Developed Reserves (Volume)	39,439	37,908	24,791
Proved Undeveloped Reserve (Volume)	12,078	9,534	2,510
Gas (Mmcf) [Member]
Estimated Proved Reserves, Beginning Balance	179,939	58,785	61,251
Acquisitions	209	115,876	8,640
Sales	(916)
Extensions, discoveries and other additions	20,247	10,241	4,664
Revisions	(10,128)	4,033	(6,678)
Production	(15,767)	(8,996)	(9,092)
Estimated Proved Reserves, Ending Balance	173,584	179,939	58,785
Proved Developed Reserves (Volume)	107,687	120,687	52,739
Proved Undeveloped Reserve (Volume)	65,897	59,252	6,046
Oil Equivalent (Mboe) [Member]
Estimated Proved Reserves, Beginning Balance	77,432	37,099	27,431
Acquisitions	401	35,742	9,427
Sales	(1,568)
Extensions, discoveries and other additions	10,729	8,095	3,043
Revisions	2,264	1,800	1,666
Production	(8,810)	(5,304)	(4,468)
Estimated Proved Reserves, Ending Balance	80,448	77,432	37,099
Proved Developed Reserves (Volume)	57,387	58,022	33,581
Proved Undeveloped Reserve (Volume)	23,061	19,409	3,518