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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2012
Table Text Block [Abstract]  
Asset Retirement Obligations Rollforward
      
millions2012 2011
Carrying amount of asset retirement obligations at January 1$1,768 $1,571
Liabilities incurred 70  39
Property dispositions (78)  (4)
Liabilities settled (89)  (68)
Accretion expense 110  100
Revisions in estimated liabilities 104  130
Carrying amount of asset retirement obligations at December 31 (1)$1,885 $1,768
      

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(1)        Short-term AROs of $298 million at December 31, 2012, and $31 million at December 31, 2011, were included in accrued expenses on the Company's Consolidated Balance Sheets.