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		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	[1]	$ 93	$ 36	$ 144	$ 61
Gross Realized Gains		2	2	17	3
Gross Realized Losses		(1)	(1)	(4)	(2)
Net Realized Gains (Losses)		1	1	13	1
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	[1]	320	154	567	331
Gross Realized Gains		32	10	53	25
Gross Realized Losses		(5)	(6)	(9)	(22)
Net Realized Gains (Losses)		$ 27	$ 4	$ 44	$ 3

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