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Asset Retirement Obligation (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Asset Retirement Obligation          
Asset retirement obligation at beginning of year     $ 3,887    
Liabilities incurred     210    
Liabilities acquired     33    
Liabilities settled     (227)    
Accretion expense 57 38 112 75  
Revisions in estimated liabilities     171    
Asset retirement obligation at end of the period 4,186   4,186    
Less current portion (447)   (447)   (447)
Asset retirement obligation - long-term $ 3,739   $ 3,739   $ 3,440