Available-For-Sale Securities (Amount Of Available-For-Sale Debt Securities By Maturity Periods) (Detail) $ in Millions	Jun. 30, 2017 USD ($)
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	$ 1
1 - 5 years	35
6 - 10 years	29
11 - 15 years	6
16 - 20 years	18
Over 20 years	111
Total Available-for-Sale Debt Securities	200
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year	29
1 - 5 years	239
6 - 10 years	223
11 - 15 years	65
16 - 20 years	66
Over 20 years	296
Total Available-for-Sale Debt Securities	$ 918