Aging of Capitalized Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Well Project	Mar. 31, 2012 Well Project	Dec. 31, 2012	Dec. 31, 2011
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 444,228	$ 563,505	$ 445,697	$ 556,412
No. of Wells	41	89
No. of Projects	11	16
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	56,324	109,907
No. of Wells	6	29
No. of Projects	3	5
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	40,721	141,441
No. of Wells	3	16
No. of Projects	1	4
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	79,446	55,922
No. of Wells	8	9
No. of Projects	2	2
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 267,737	$ 256,235
No. of Wells	24	35
No. of Projects	5	5