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Asset Retirement Obligation - Asset Retirement Obligation (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]          
Asset retirement obligation at December 31, 2012     $ 4,578    
Liabilities incurred     251    
Liabilities acquired     53    
Liabilities settled     (270)    
Accretion expense 65 57 130 112  
Revisions in estimated liabilities     11    
Asset retirement obligation at June 30, 2013 4,753   4,753    
Less current portion (475)   (475)   (478)
Asset retirement obligation, long-term $ 4,278   $ 4,278   $ 4,100