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		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	[1]	$ 24	$ 20	$ 168	$ 81
Gross Realized Gains		0	2	17	5
Gross Realized Losses		(1)	(2)	(5)	(4)
Net Realized Gains (Losses)		(1)	0	12	1
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	[1]	278	139	845	470
Gross Realized Gains		29	11	82	36
Gross Realized Losses		(5)	(3)	(14)	(25)
Net Realized Gains (Losses)		$ 24	$ 8	$ 68	$ 11

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