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Asset Retirement Obligations Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Asset Retirement Obligation Begining Balance $ 1,783  
Asset Retirement Obligation, Liabilities Incurred 10  
Asset Retirement Obligation, Liabilities Settled (41)  
Asset Retirement Obligation, Accretion Expense 103  
Asset Retirement Obligation, Revision of Estimate 306  
Asset Retirement Obligation, Held for Sale (41)  
Asset Retirement Obligation Ending Balance 2,120 [1]  
Asset Retirement Obligation, Current $ 93 $ 34
[1] (a) Beginning and ending balances include asset retirement obligations of $34 million and $93 million classified as short-term at December 31, 2012 and September 30, 2013.