Available-For-Sale Securities (Narrative) (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Facility	Jun. 30, 2014 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other-Than Temporary Impairments	$ 10	$ 2	$ 15	$ 4
Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of Nuclear Facilities | Facility			5
Other-Than Temporary Impairments	$ 10	$ 2	$ 15	$ 4
Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-Than Temporary Impairments			15
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			111
Rabbi Trust [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI			$ 6