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Summary of Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2012
sqmi
Dec. 31, 2011
Accounting Policies [Abstract]    
Supply of electricity, area covered (square mile) 50,000  
Cash Equivalents    
Money market funds $ 28 $ 21
Maximum maturity period of cash equivalent investments (in months) 3 months  
Cash reclassified to accounts payable under cash management program 233 220
Inventory    
Fuel 19 24
Materials and supplies, spare parts 319 326
Total inventory $ 338 $ 350