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, 2017	Mar. 31, 2016
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	[1]	$ 51	$ 25
Gross Realized Gains		15	1
Gross Realized Losses		(3)	(1)
Net Realized Gains (Losses)		12	0
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	[1]	247	177
Gross Realized Gains		21	15
Gross Realized Losses		(4)	(16)
Net Realized Gains (Losses)		$ 17	$ (1)

[1] Includes activity in accounts related to the liquidation of funds being transitioned to new managers.