Available-For-Sale Securities (Narrative) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Facility	Sep. 30, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other-Than-Temporary Impairments	$ 5	$ 30	$ 25	$ 45
Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of Nuclear Facilities | Facility			5
Other-Than-Temporary Impairments	$ 5	$ 30	$ 25	$ 45
Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-Than-Temporary Impairments			25
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI			131
Rabbi Trust [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI			$ 10