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Asset Retirement Obligations (Details) (USD $)
In Millions
3 Months Ended9 Months Ended
Mar. 31, 2011
Sep. 30, 2011
Dec. 31, 2010
Asset Retirement Obligations [Line Items]   
Asset retirement obligations at December 31, 2010$ 1,514$ 1,514 
Accretion 69 
Liabilities incurred 3 
Revision in estimated cash flows19(1) 
Liabilities settled (15) 
Asset retirement obligations at September 30, 2011 1,570 
Less amount included in current liabilities (10) 
Asset retirement obligations, net $ 1,560$ 1,498