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Asset Retirement Obligations Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Asset Retirement Obligation Begining Balance $ 1,783
Asset Retirement Obligation, Liabilities Incurred 8
Asset Retirement Obligation, Liabilities Settled (27)
Asset Retirement Obligation, Accretion Expense 48
Asset Retirement Obligation, Revision of Estimate 306
Asset Retirement Obligation, Held for Sale (39)
Asset Retirement Obligation Ending Balance 2,079 [1]
Asset Retirement Obligation, Current $ 40
[1] (a) Includes asset retirement obligations of $40 million classified as a short-term at June 30, 2013