Available-For-Sale Securities (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Mar. 31, 2016 USD ($)
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	$ 9
1 - 5 years	42
6 - 10 years	47
11 - 15 years	5
16 - 20 years	9
Over 20 years	80
Total Available-for-Sale Debt Securities	192
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	24
1 - 5 years	215
6 - 10 years	209
11 - 15 years	57
16 - 20 years	53
Over 20 years	316
Total Available-for-Sale Debt Securities	$ 874