Available-For-Sale Securities (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Sep. 30, 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	48
11 - 15 years	9
16 - 20 years	9
Over 20 years	84
Total Available-for-Sale Debt Securities	201
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	22
1 - 5 years	233
6 - 10 years	214
11 - 15 years	56
16 - 20 years	62
Over 20 years	303
Total Available-for-Sale Debt Securities	$ 890