Trust Investments (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Less than one year	$ 1
1 - 5 years	39
6 - 10 years	23
11 - 15 years	7
16 - 20 years	18
Over 20 years	112
Total Available-for-Sale Debt Securities	200
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Trust Investments [Line Items]
Less than one year	10
1 - 5 years	298
6 - 10 years	196
11 - 15 years	45
16 - 20 years	71
Over 20 years	347
Total Available-for-Sale Debt Securities	$ 967