Trust Investments (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Less than one year	$ 1
1 - 5 years	36
6 - 10 years	29
11 - 15 years	6
16 - 20 years	17
Over 20 years	113
Total Available-for-Sale Debt Securities	202
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Trust Investments [Line Items]
Less than one year	15
1 - 5 years	300
6 - 10 years	201
11 - 15 years	40
16 - 20 years	73
Over 20 years	335
Total Available-for-Sale Debt Securities	$ 964