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Available-for-Sale Securities (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Dec. 31, 2010
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost $ 487   $ 479  
Gross unrealized gains 232   187  
Gross unrealized losses 2   2  
Fair value 717   664  
Available-for-Sale Securities Proceeds From and Realized Gains and Losses (Details) [Abstract]        
Proceeds from sales of NDT securities 34 75    
Other proceeds from sales 0 163    
Gross realized gains 6 [1] 17 [1]    
Gross realized losses 1 [1] 5 [1]    
Short Term Investments (Numeric) [Abstract]        
Principal amount of tax-exempt revenue bonds held as short-term investment       163
Debt Securities [Member]
       
Available-for-Sale Securities Scheduled Maturity Dates of Debt Securities (Details) [Abstract]        
Amortized cost - maturity less than 1 year 9      
Amortized cost - maturity between 1 and 5 years 77      
Amortized cost - maturity between 5 and 10 years 64      
Amortized cost - maturity in excess of 10 years 81      
Amortized cost total 231      
Fair value - maturity less than 1 year 9      
Fair value - maturity between 1 and 5 years 80      
Fair value - maturity between 5 and 10 years 70      
Fair value - maturity in excess of 10 years 86      
Fair value total 245      
Nuclear Decommissioning Trust Funds [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 462   454  
Gross unrealized gains 232   187  
Gross unrealized losses 1   1  
Fair value 693   640  
Nuclear Decommissioning Trust Funds [Member] | Cash and Cash Equivalents [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 8   12  
Gross unrealized gains 0   0  
Gross unrealized losses 0   0  
Fair value 8   12  
Nuclear Decommissioning Trust Funds [Member] | Equity Securities [Member] | US Large Cap [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 177   173  
Gross unrealized gains 152   119  
Gross unrealized losses 0   0  
Fair value 329   292  
Nuclear Decommissioning Trust Funds [Member] | Equity Securities [Member] | US Mid Small Cap [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 69   67  
Gross unrealized gains 64   50  
Gross unrealized losses 0   0  
Fair value 133   117  
Nuclear Decommissioning Trust Funds [Member] | Debt Securities [Member] | US Treasury [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 79   76  
Gross unrealized gains 8   10  
Gross unrealized losses 0   0  
Fair value 87   86  
Nuclear Decommissioning Trust Funds [Member] | Debt Securities [Member] | US Government Sponsored Agency [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 9   9  
Gross unrealized gains 1   1  
Gross unrealized losses 0   0  
Fair value 10   10  
Nuclear Decommissioning Trust Funds [Member] | Debt Securities [Member] | Municipality [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 81   80  
Gross unrealized gains 4   4  
Gross unrealized losses 1   1  
Fair value 84   83  
Nuclear Decommissioning Trust Funds [Member] | Debt Securities [Member] | Investment Grade Corporate [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 35   35  
Gross unrealized gains 3   3  
Gross unrealized losses 0   0  
Fair value 38   38  
Nuclear Decommissioning Trust Funds [Member] | Debt Securities [Member] | Other [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 2   2  
Gross unrealized gains 0   0  
Gross unrealized losses 0   0  
Fair value 2   2  
Nuclear Decommissioning Trust Funds [Member] | Receivables Payables Net [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 2   0  
Gross unrealized gains 0   0  
Gross unrealized losses 0   0  
Fair value 2   0  
Auction Rate Securities [Member]
       
Available-for-Sale Securities Balance Sheet (Details) [Abstract]        
Amortized cost 25   25  
Gross unrealized gains 0   0  
Gross unrealized losses 1   1  
Fair value $ 24   $ 24  
[1] Excludes the impact of other-than-temporary impairment charges recognized in the Statements of Income.