Aging of Capitalized Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Well Project	Dec. 31, 2010 Well Project	Dec. 31, 2009 Well Project	Dec. 31, 2008
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 556,412	$ 497,765	$ 369,862	$ 310,118
No. of Wells	68	59	46
No. of Projects	16	15	14
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	69,757	135,494	117,618
No. of Wells	11	15	10
No. of Projects	5	4	6
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	143,611	115,418	49,628
No. of Wells	15	10	4
No. of Projects	3	4	4
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	101,696	42,571	8,870
No. of Wells	9	3	5
No. of Projects	2	3	0
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 241,348	$ 204,282	$ 193,746
No. of Wells	33	31	27
No. of Projects	6	4	4