Summary of Estimated Proved Oil and Natural Gas Reserve (Detail)	12 Months Ended
	Dec. 31, 2013 MBbls	Dec. 31, 2012 MBbls	Dec. 31, 2011 MBbls
Oil (MBbls) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	44,918	45,655	33,203
Revisions of previous estimates	3,606	(1,559)	2,889
Extensions, discoveries and other additions	2,367	3,681	3,544
Purchase of producing properties		4,336	14,396
Sale of reserves	(170)	(60)	(1,950)
Production	(6,894)	(7,135)	(6,427)
Estimated proved reserves, ending balance	43,827	44,918	45,655
Estimated proved developed reserves	27,920	29,005	30,914
Natural Gas Liquids (MBbls) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	18,066	4,405
Revisions of previous estimates	2,439	9,349	4,911
Extensions, discoveries and other additions	4,395	4,856
Purchase of producing properties		619
Production	(1,603)	(1,163)	(506)
Estimated proved reserves, ending balance	23,297	18,066	4,405
Estimated proved developed reserves	11,569	8,593	3,195
Natural Gas (MMcf) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	395,374	325,479	274,705
Revisions of previous estimates	36,006	(26,694)	(26,725)
Extensions, discoveries and other additions	79,729	131,408	93,520
Purchase of producing properties		8,168	24,595
Sale of reserves	(214)	(418)	(2,150)
Production	(50,129)	(42,569)	(38,466)
Estimated proved reserves, ending balance	460,766	395,374	325,479
Estimated proved developed reserves	246,946	210,956	171,871
Oil And Natural Gas (MMcfe) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	773,285	625,839	473,923
Revisions of previous estimates	72,275	20,050	20,075
Extensions, discoveries and other additions	120,299	182,633	114,784
Purchase of producing properties		37,895	110,971
Sale of reserves	(1,235)	(775)	(13,850)
Production	(101,111)	(92,357)	(80,064)
Estimated proved reserves, ending balance	863,513	773,285	625,839
Estimated proved developed reserves	483,885	436,540	376,528