Available-For-Sale Securities (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	$ 0
1 - 5 years	40
6 - 10 years	27
11 - 15 years	6
16 - 20 years	19
Over 20 years	112
Total Available-for-Sale Debt Securities	204
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	37
1 - 5 years	236
6 - 10 years	230
11 - 15 years	62
16 - 20 years	67
Over 20 years	293
Total Available-for-Sale Debt Securities	$ 925