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Asset Retirement Obligation - Asset Retirement Obligation (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Asset Retirement Obligation [Line Items]      
Asset retirement obligation at beginning of year $ 3,887 $ 2,872  
Liabilities incurred 592 419  
Liabilities acquired 72 592  
Liabilities settled (550) (549)  
Accretion expense 232 154 111
Revisions in estimated liabilities 345 399  
Asset retirement obligation at end of year 4,578 3,887 2,872
Less current portion (478) (447)  
Asset retirement obligation, long-term $ 4,100 $ 3,440