Changes in Capitalized Exploratory Drilling Costs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Well	Dec. 31, 2010 Well	Dec. 31, 2009 Well
Number of Wells
Balance at beginning of year	1		1
Additions pending determination of proved reserves		1
Reclassification to wells, equipment and facilities based on the determination of proved reserves			(1)
Charged to exploration expense	(1)
Balance at end of year		1
Cost
Balance at beginning of year	$ 6,180		$ 2,482
Additions pending determination of proved reserves		6,180
Reclassification to wells, equipment and facilities based on the determination of proved reserves			(2,482)
Charged to exploration expense	(6,180)
Balance at end of year		$ 6,180