Suspended Exploratory Well Costs - Suspended Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013 Project	Dec. 31, 2012 Project
Exploratory Wells Drilled [Line Items]
Balance at beginning of period	$ 57,360	$ 93,388
Additions to capitalized exploratory well costs pending the determination of proved reserves	62,640	153,250
Reclassifications to wells, facilities and equipment based on determination of proved reserves	(58,601)	(184,298)
Divested wells		(4,980)
Balance at end of period	61,399	57,360
Less exploratory well costs that have been capitalized for a period of one year or less	(39,140)	(45,965)
Capitalized exploratory well costs that have been capitalized for more than one year	$ 22,259	$ 11,395
Number of projects that have exploratory well costs that have been capitalized for a period greater than one year	9	5