Oil and Gas Reserves Information (Unaudited) - Summary of Changes in Net Quantities of Crude Oil and Natural Gas Reserves For Continuing Operations (Detail)	12 Months Ended
Dec. 31, 2014 MBbls	Dec. 31, 2013 MBbls	Dec. 31, 2012 MBbls
Oil (MBbls)
Proved Reserves:
Beginning of year	21,976	18,899	13,234
Revisions of previous estimates	(2,182)	28	327
Extensions and discoveries	5,373	5,363	11,953
Sales of minerals in place			(4,823)
Production	(4,313)	(2,314)	(1,792)
End of year	20,854	21,976	18,899
Proved Developed Reserves:
Beginning of year	13,914	8,389	6,499
End of year	16,247	13,914	8,389
Natural Gas (MMcf)
Proved Reserves:
Beginning of year	452,653	437,445	1,080,644
Revisions of previous estimates	3,998	23,321	(529,272)
Extensions and discoveries	78,383	47,581	21,525
Sales of minerals in place			(53,690)
Production	(39,768)	(55,694)	(81,762)
End of year	495,266	452,653	437,445
Proved Developed Reserves:
Beginning of year	344,278	362,426	546,627
End of year	324,597	344,278	362,426

Proved developed reserves.

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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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Production of proved reserves.

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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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Sales of minerals in place.

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