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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 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost $ 1,802 $ 1,802 $ 1,794
Gross Unrealized Gains 347 347 350
Gross Unrealized Losses (53) (53) (11)
Fair Value 2,096 2,096 2,133
Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
Cost 228 228 227
Gross Unrealized Gains 4 4 6
Gross Unrealized Losses (7) (7) (2)
Fair Value 225 225 231
Rabbi Trust [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Fair Value 57 57 57
Domestic Equity Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost 467 467 497
Gross Unrealized Gains 268 268 245
Gross Unrealized Losses (7) (7) (2)
Fair Value 728 728 740
Domestic Equity Securities [Member] | Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
Cost 21 21 24
Gross Unrealized Gains 4 4 3
Gross Unrealized Losses 0 0 0
Fair Value 25 25 27
International Equity Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost 330 330 311
Gross Unrealized Gains 78 78 99
Gross Unrealized Losses (16) (16) (3)
Fair Value 392 392 407
International Equity Securities [Member] | Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
Cost 0 0 0
Gross Unrealized Gains 0 0 0
Gross Unrealized Losses 0 0 0
Fair Value 0 0 0
Equity Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost 797 797 808
Gross Unrealized Gains 346 346 344
Gross Unrealized Losses (23) (23) (5)
Fair Value 1,120 1,120 1,147
Unrealized Gains (Losses) on Equity Securities still held 41 26  
Equity Securities [Member] | Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
Cost 21 21 24
Gross Unrealized Gains 4 4 3
Gross Unrealized Losses 0 0 0
Fair Value 25 25 27
Unrealized Gains (Losses) on Equity Securities still held 1 2  
Debt Securities [Member] | Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
After tax amount of net unrealized gains recognized in AOCI   (5)  
Government Debt Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost 537 537 586
Gross Unrealized Gains 0 0 2
Gross Unrealized Losses (17) (17) (4)
Fair Value 520 520 584
Government Debt Securities [Member] | Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
Cost 103 103 85
Gross Unrealized Gains 0 0 1
Gross Unrealized Losses (4) (4) (1)
Fair Value 99 99 85
Corporate Debt Obligations [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost 468 468 400
Gross Unrealized Gains 1 1 4
Gross Unrealized Losses (13) (13) (2)
Fair Value 456 456 402
Corporate Debt Obligations [Member] | Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
Cost 104 104 118
Gross Unrealized Gains 0 0 2
Gross Unrealized Losses (3) (3) (1)
Fair Value 101 101 119
Total Debt Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost 1,005 1,005 986
Gross Unrealized Gains 1 1 6
Gross Unrealized Losses (30) (30) (6)
Fair Value 976 976 986
After tax amount of net unrealized gains recognized in AOCI   (17)  
Total Debt Securities [Member] | Rabbi Trust [Member]      
Schedule of Trust Investments [Line Items]      
Cost 207 207 203
Gross Unrealized Gains 0 0 3
Gross Unrealized Losses (7) (7) (2)
Fair Value 200 200 $ 204
Other Securities [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Trust Investments [Line Items]      
Cost 0 0  
Gross Unrealized Gains 0 0  
Gross Unrealized Losses 0 0  
Fair Value $ 0 $ 0