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Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Dec. 31, 2013
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax $ 1 $ (11)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax (3) 5  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (2) (6)  
Cash Flow Hedges [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Energy-Related Contracts, Pre-Tax (12) 6 12
Energy-Related Contracts, Tax 5 (2)  
Energy-Related Contracts, After-Tax (7) 4  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (7) 4 7
Pension and OPEB Plans [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax (6) (17)  
Reclassification Adjustment from AOCI, Pension and OPEB, Tax 2 7  
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax (4) (10)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (4) (10)  
Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax 19 0  
Reclassification for Available for Sale Securities, Tax (10) 0  
Reclassification for Available for Sale Securities, After-Tax 9 0  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 9 0  
Power [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax 1 (8)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax (3) 3  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (2) (5)  
Power [Member] | Cash Flow Hedges [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Energy-Related Contracts, Pre-Tax (12) 6  
Energy-Related Contracts, Tax 5 (2)  
Energy-Related Contracts, After-Tax (7) 4  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (7) 4  
Power [Member] | Pension and OPEB Plans [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax (4) (14)  
Reclassification Adjustment from AOCI, Pension and OPEB, Tax 1 5  
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax (3) (9)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (3) (9)  
Power [Member] | Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax 17 0  
Reclassification for Available for Sale Securities, Tax (9) 0  
Reclassification for Available for Sale Securities, After-Tax 8 0  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 8 0  
Operating Expense [Member] | Pension and OPEB Plans [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Amortization of Prior Service (Cost) Credit, Pre-Tax 2 4  
Amortization of Prior Service (Cost) Credit, Tax (1) (2)  
Amortization of Prior Service (Cost) Credit, After-Tax 1 2  
Amortization of Actuarial Loss, Pre-Tax (8) (21)  
Amortization of Actuarial Loss, Tax 3 9  
Amortization of Actuarial Loss, After-Tax (5) (12)  
Operating Expense [Member] | Power [Member] | Pension and OPEB Plans [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Amortization of Prior Service (Cost) Credit, Pre-Tax 2 2  
Amortization of Prior Service (Cost) Credit, Tax (1) (1)  
Amortization of Prior Service (Cost) Credit, After-Tax 1 1  
Amortization of Actuarial Loss, Pre-Tax (6) (16)  
Amortization of Actuarial Loss, Tax 2 6  
Amortization of Actuarial Loss, After-Tax (4) (10)  
Operating Revenues [Member] | Cash Flow Hedges [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Energy-Related Contracts, Pre-Tax (12) 6  
Energy-Related Contracts, Tax 5 (2)  
Energy-Related Contracts, After-Tax (7) 4  
Operating Revenues [Member] | Power [Member] | Cash Flow Hedges [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Energy-Related Contracts, Pre-Tax (12) 6  
Energy-Related Contracts, Tax 5 (2)  
Energy-Related Contracts, After-Tax (7) 4  
Other Income [Member] | Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax 25 2  
Reclassification for Available for Sale Securities, Tax (13) (1)  
Reclassification for Available for Sale Securities, After-Tax 12 1  
Other Income [Member] | Power [Member] | Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax 23 2  
Reclassification for Available for Sale Securities, Tax (12) (1)  
Reclassification for Available for Sale Securities, After-Tax 11 1  
Other Expense [Member] | Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax (4) 0  
Reclassification for Available for Sale Securities, Tax 2 0  
Reclassification for Available for Sale Securities, After-Tax (2) 0  
Other Expense [Member] | Power [Member] | Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax (4) 0  
Reclassification for Available for Sale Securities, Tax 2 0  
Reclassification for Available for Sale Securities, After-Tax (2) 0  
Other-Than-Temporary Impairments [Member] | Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax (2) (2)  
Reclassification for Available for Sale Securities, Tax 1 1  
Reclassification for Available for Sale Securities, After-Tax (1) (1)  
Other-Than-Temporary Impairments [Member] | Power [Member] | Available-for-Sale Securities [Member]
      
Reclassification out of Accumulated Other Comprehensive Income [Line Items]      
Reclassification for Available for Sale Securities, Pre-Tax (2) (2)  
Reclassification for Available for Sale Securities, Tax 1 1  
Reclassification for Available for Sale Securities, After-Tax $ (1) $ (1)