Available-For-Sale Securities (Proceeds From The Sales Of And The Net Realized Gains On Securities in the NDT and Rabbi Trusts) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	$ 25	$ 19	[1]
Gross Realized Gains	1	0
Gross Realized Losses	(1)	0
Net Realized Gains (Losses)	0	0
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	177	590	[2]
Gross Realized Gains	15	19
Gross Realized Losses	(16)	(9)
Net Realized Gains (Losses)	$ (1)	$ 10

[1] 2015 proceeds include activity in accounts related to the liquidation of funds being transitioned to new managers

[2] 2015 proceeds include activity in accounts related to the liquidation of funds being transitioned to new managers.