Aging of Capitalized Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 Well Project	Sep. 30, 2011 Well Project	Dec. 31, 2011	Dec. 31, 2010
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 571,776	$ 529,246	$ 556,412	$ 497,765
No. of Wells	47	69
No. of Projects	15	16
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	82,521	92,752
No. of Wells	8	15
No. of Projects	5	5
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	90,390	69,591
No. of Wells	7	9
No. of Projects	3	1
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	114,532	115,924
No. of Wells	6	8
No. of Projects	1	3
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount	$ 284,333	$ 250,979
No. of Wells	26	37
No. of Projects	6	7