Supplemental Information on Oil and Natural Gas Operations - Unaudited - Summary of Estimated Proved Oil and Natural Gas Reserve (Detail)	12 Months Ended
Dec. 31, 2014 MBbls	Dec. 31, 2013 MBbls	Dec. 31, 2012 MBbls
Oil (MBbls) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	43,827	44,918	45,655
Revisions of previous estimates	(624)	3,606	(1,559)
Extensions, discoveries and other additions	9,650	2,367	3,681
Purchase of producing properties			4,336
Sale of reserves	(4,888)	(170)	(60)
Production	(5,568)	(6,894)	(7,135)
Estimated proved reserves, ending balance	42,397	43,827	44,918
Estimated proved developed reserves	22,957	27,920	29,005
Estimated proved undeveloped reserves	19,440	15,907	15,913
Natural Gas Liquids (MBbls) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	23,297	18,066	4,405
Revisions of previous estimates	(331)	2,439	9,349
Extensions, discoveries and other additions	7,521	4,395	4,856
Purchase of producing properties			619
Sale of reserves	(556)
Production	(2,114)	(1,603)	(1,163)
Estimated proved reserves, ending balance	27,817	23,297	18,066
Estimated proved developed reserves	13,743	11,569	8,593
Estimated proved undeveloped reserves	14,074	11,728	9,473
Natural Gas (MMcfe) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	460,766	395,374	325,479
Revisions of previous estimates	(4,631)	36,006	(26,694)
Extensions, discoveries and other additions	131,617	79,729	131,408
Purchase of producing properties			8,168
Sale of reserves	(46,483)	(214)	(418)
Production	(47,426)	(50,129)	(42,569)
Estimated proved reserves, ending balance	493,843	460,766	395,374
Estimated proved developed reserves	249,924	246,946	210,956
Estimated proved undeveloped reserves	243,919	213,820	184,418
Oil And Natural Gas (MMcfe) [Member]
Reserve Quantities [Line Items]
Estimated proved reserves, beginning balance	863,513	773,285	625,839
Revisions of previous estimates	(10,362)	72,275	20,050
Extensions, discoveries and other additions	234,639	120,299	182,633
Purchase of producing properties			37,895
Sale of reserves	(79,151)	(1,235)	(775)
Production	(93,515)	(101,111)	(92,357)
Estimated proved reserves, ending balance	915,124	863,513	773,285
Estimated proved developed reserves	470,118	483,885	436,540
Estimated proved undeveloped reserves	445,006	379,628	336,745

Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

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The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

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Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors.

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