Aging of Capitalized Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 Well Project	Jun. 30, 2012 Well Project	Dec. 31, 2012	Dec. 31, 2011
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 444,428	$ 567,645	$ 445,697	$ 556,412
No. of Wells	41	97
No. of Projects	10	16
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	49,994	103,807
No. of Wells	3	36
No. of Projects	1	6
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	37,898	103,141
No. of Wells	5	15
No. of Projects	1	3
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	73,863	67,197
No. of Wells	7	9
No. of Projects	3	2
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 282,673	$ 293,500
No. of Wells	26	37
No. of Projects	5	5