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Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2011
Jun. 30, 2011
Dec. 31, 2011
Dec. 31, 2010
Asset Retirement Obligations [Line Items]        
Asset retirement obligations at January 1, 2011 $ 1,514 $ 1,514 $ 1,514  
Accretion     93  
Liabilities incurred     10  
Liabilities settled     (23)  
Revision in estimated cash flows 19 20 (1) 22
Asset retirement obligations at December 31, 2011     1,593 1,514
Less amount included in current liabilities     (2)  
Asset retirement obligations, net     $ 1,591 $ 1,498