Available-For-Sale Securities (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Mar. 31, 2017 USD ($)
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	$ 6
1 - 5 years	50
6 - 10 years	42
11 - 15 years	9
16 - 20 years	8
Over 20 years	82
Total Available-for-Sale Debt Securities	197
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	16
1 - 5 years	257
6 - 10 years	199
11 - 15 years	59
16 - 20 years	63
Over 20 years	266
Total Available-for-Sale Debt Securities	$ 860