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Available-For-Sale Securities (Narrative) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]      
Available For Sale Securities OTTI Charge $ 12 $ 28 $ 53
Power [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available For Sale Securities OTTI Charge 12 $ 28 $ 53
Nuclear Decommissioning Trust (NDT) Fund [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available For Sale Securities OTTI Charge 12    
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent 175    
Decommissioning Liability, Noncurrent 756    
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Minimum [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Decommissioning Costs Including Contingencies 2,800    
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Maximum [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Decommissioning Costs Including Contingencies 3,000    
Debt Securities [Member] | Rabbi Trust [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent $ 2