Available-For-Sale Securities (Narrative) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Facility	Sep. 30, 2014 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other-Than Temporary Impairments	$ 30	$ 10	$ 45	$ 14
Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of Nuclear Facilities | Facility			5
Other-Than Temporary Impairments	$ 30	$ 10	$ 45	$ 14
Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-Than Temporary Impairments			45
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			82
Rabbi Trust [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI			$ 4