Available-For-Sale Securities (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Other-Than Temporary Impairments	$ 5	$ 2
Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-Than Temporary Impairments	5	2
Number of Nuclear Facilities	5
Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-Than Temporary Impairments	5
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	123
Rabbi Trust [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI	$ 9