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Aging of Capitalized Exploratory Well Costs (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2011
Well
Bushel
Project
Location
Sep. 30, 2010
Well
Project
Bushel
Dec. 31, 2010
Dec. 31, 2009
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]    
Amount$ 529,246$ 459,659$ 497,765$ 369,862
No. of Wells6961  
No. of Projects1615  
Zero to one year
    
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]    
Amount92,75283,642  
No. of Wells1513  
No. of Projects55  
One to two years
    
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]    
Amount69,591118,776  
No. of Wells912  
No. of Projects13  
Two to three years
    
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]    
Amount115,92450,604  
No. of Wells84  
No. of Projects34  
Three years or more
    
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]    
Amount$ 250,979$ 206,637  
No. of Wells3732  
No. of Projects73