Trust Investments (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Mar. 31, 2019 USD ($)
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Less than one year	$ 0
1 - 5 years	33
6 - 10 years	30
11 - 15 years	10
16 - 20 years	25
Over 20 years	110
Total Available-for-Sale Debt Securities	208
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Trust Investments [Line Items]
Less than one year	13
1 - 5 years	257
6 - 10 years	215
11 - 15 years	44
16 - 20 years	71
Over 20 years	409
Total Available-for-Sale Debt Securities	$ 1,009