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Trust Investments (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]    
Schedule of Trust Investments [Line Items]    
Cost $ 1,764 $ 1,794
Gross Unrealized Gains 321 350
Gross Unrealized Losses (34) (11)
Fair Value 2,051 2,133
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Domestic Equity Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 463 497
Gross Unrealized Gains 226 245
Gross Unrealized Losses (7) (2)
Fair Value 682 740
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | International Equity Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 318 311
Gross Unrealized Gains 93 99
Gross Unrealized Losses (7) (3)
Fair Value 404 407
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 781 808
Gross Unrealized Gains 319 344
Gross Unrealized Losses (14) (5)
Fair Value 1,086 1,147
Unrealized Gains (Losses) on Equity Securities still held (10)  
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government [Member]    
Schedule of Trust Investments [Line Items]    
Cost 518 586
Gross Unrealized Gains 1 2
Gross Unrealized Losses (11) (4)
Fair Value 508 584
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Corporate [Member]    
Schedule of Trust Investments [Line Items]    
Cost 464 400
Gross Unrealized Gains 1 4
Gross Unrealized Losses (9) (2)
Fair Value 456 402
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]    
Schedule of Trust Investments [Line Items]    
After tax amount of net unrealized gains recognized in AOCI (10)  
Cost 982 986
Gross Unrealized Gains 2 6
Gross Unrealized Losses (20) (6)
Fair Value 964 986
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 1  
Gross Unrealized Gains 0  
Gross Unrealized Losses 0  
Fair Value 1  
Rabbi Trust [Member]    
Schedule of Trust Investments [Line Items]    
Cost 226 227
Gross Unrealized Gains 3 6
Gross Unrealized Losses (4) (2)
Fair Value 225 231
Rabbi Trust [Member] | Domestic Equity Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 21 24
Gross Unrealized Gains 2 3
Gross Unrealized Losses 0 0
Fair Value 23 27
Rabbi Trust [Member] | International Equity Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 0 0
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 0 0
Rabbi Trust [Member] | Equity Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 21 24
Gross Unrealized Gains 2 3
Gross Unrealized Losses 0 0
Fair Value 23 27
Unrealized Gains (Losses) on Equity Securities still held (1)  
Rabbi Trust [Member] | Debt Securities [Member]    
Schedule of Trust Investments [Line Items]    
After tax amount of net unrealized gains recognized in AOCI (3)  
Rabbi Trust [Member] | Government [Member]    
Schedule of Trust Investments [Line Items]    
Cost 91 85
Gross Unrealized Gains 0 1
Gross Unrealized Losses (2) (1)
Fair Value 89 85
Rabbi Trust [Member] | Corporate [Member]    
Schedule of Trust Investments [Line Items]    
Cost 114 118
Gross Unrealized Gains 1 2
Gross Unrealized Losses (2) (1)
Fair Value 113 119
Rabbi Trust [Member] | Total Debt Securities [Member]    
Schedule of Trust Investments [Line Items]    
Cost 205 203
Gross Unrealized Gains 1 3
Gross Unrealized Losses (4) (2)
Fair Value 202 204
Rabbi Trust [Member] | Power [Member]    
Schedule of Trust Investments [Line Items]    
Fair Value $ 56 $ 57