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Asset Retirement Obligation - Asset Retirement Obligation (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]          
Asset retirement obligation at December 31, 2012     $ 4,578    
Liabilities incurred     383    
Liabilities acquired     53    
Liabilities divested     (1,563)    
Liabilities settled     (396)    
Accretion expense 66 60 196 172  
Revisions in estimated liabilities     11    
Asset retirement obligation at September 30, 2013 3,262   3,262    
Less current portion (139)   (139)   (478)
Asset retirement obligation, long-term $ 3,123   $ 3,123   $ 4,100