Aging of Capitalized Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011 Well Bushel Project	Dec. 31, 2010	Jun. 30, 2010 Bushel Project Well	Dec. 31, 2009
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 532,903	$ 497,765	$ 430,424	$ 369,862
No. of Wells	67		58
No. of Projects	16		15
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	116,514		103,705
No. of Wells	16		14
No. of Projects	5		5
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	96,709		102,446
No. of Wells	11		10
No. of Projects	2		4
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	104,420		17,946
No. of Wells	8		2
No. of Projects	4		2
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 215,260		$ 206,327
No. of Wells	32		32
No. of Projects	5		4