Trust Investments (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Less than one year	$ 3
1 - 5 years	28
6 - 10 years	32
11 - 15 years	7
16 - 20 years	22
Over 20 years	108
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	11
1 - 5 years	282
6 - 10 years	201
11 - 15 years	47
16 - 20 years	73
Over 20 years	362
Total Available-for-Sale Debt Securities	$ 976