Available-For-Sale Securities (Narrative) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) Facility	Mar. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other-Than-Temporary Impairments	$ 1	$ 10
Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of Nuclear Facilities | Facility	5
Other-Than-Temporary Impairments	$ 1	$ 10
Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-Than-Temporary Impairments	1
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	149
Rabbi Trust [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI	$ (1)