Aging of Capitalized Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Well Project	Dec. 31, 2011 Well Project	Dec. 31, 2010 Well Project	Dec. 31, 2009
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 445,697	$ 556,412	$ 497,765	$ 369,862
No. of Wells	44	68	59
No. of Projects	15	16	15
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	59,833	69,757	135,494
No. of Wells	7	11	15
No. of Projects	2	5	4
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	18,335	143,611	115,418
No. of Wells	2	15	10
No. of Projects	3	3	4
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	83,314	101,696	42,571
No. of Wells	9	9	3
No. of Projects	4	2	3
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 284,215	$ 241,348	$ 204,282
No. of Wells	26	33	31
No. of Projects	6	6	4