Suspended Exploratory Well Costs - Suspended Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012 Project	Dec. 31, 2011 Project
Balance at beginning of period	$ 93,388	$ 23,908
Additions to capitalized exploratory well costs pending the determination of proved reserves	186,087	86,996
Reclassifications to wells, facilities and equipment based on determination of proved reserves	(178,422)	(17,516)
Divested wells	(4,980)
Balance at end of period	96,073	93,388
Less exploratory well costs that have been capitalized for a period of one year or less	(74,269)	(83,860)
Capitalized exploratory well costs that have been capitalized for a period greater than one year	$ 21,804	$ 9,528
Number of projects that have exploratory well costs that have been capitalized for a period greater than one year	8	3