Estimated Proved Oil and Natural Gas Reserves (Detail)	12 Months Ended
	Dec. 31, 2012 MMcf	Dec. 31, 2011 MMcf	Dec. 31, 2010 MMcf
Natural Gas (MMcf)
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	325,479	274,705	216,694
Revisions of previous estimates	(26,694)	(26,725)	12,670
Extensions, discoveries and other additions	131,408	93,520	82,846
Purchase of producing properties	8,168	24,595	3,816
Sale of reserves	(418)	(2,150)	(153)
Production	(42,569)	(38,466)	(41,168)
Estimated proved reserves, ending balance	395,374	325,479	274,705
Estimated proved developed reserves	210,956	171,871	174,876
Oil And Natural Gas (MMcfe)
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	625,839	473,923	410,711
Revisions of previous estimates	20,050	20,075	35,906
Extensions, discoveries and other additions	182,633	114,784	98,854
Purchase of producing properties	37,895	110,971	7,637
Sale of reserves	(775)	(13,850)	(289)
Production	(92,357)	(80,064)	(78,896)
Estimated proved reserves, ending balance	773,285	625,839	473,923
Estimated proved developed reserves	436,540	376,528	324,876
Oil (MBbls)
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	45,655	33,203	32,336
Revisions of previous estimates	(1,559)	2,889	3,299
Extensions, discoveries and other additions	3,681	3,544	2,668
Purchase of producing properties	4,336	14,396	637
Sale of reserves	(60)	(1,950)	(23)
Production	(7,135)	(6,427)	(5,714)
Estimated proved reserves, ending balance	44,918	45,655	33,203
Estimated proved developed reserves	29,005	30,914	25,000
Natural Gas Liquids (MBbls)
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	4,405
Revisions of previous estimates	9,349	4,911	574
Extensions, discoveries and other additions	4,856
Purchase of producing properties	619
Production	(1,163)	(506)	(574)
Estimated proved reserves, ending balance	18,066	4,405
Estimated proved developed reserves	8,593	3,195