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, 2016	Sep. 30, 2015		Sep. 30, 2016	Sep. 30, 2015
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	$ 20	$ 20		$ 81	$ 83
Gross Realized Gains	2	0		5	2
Gross Realized Losses	(2)	(1)		(4)	(1)
Net Realized Gains (Losses)	0	(1)		1	1
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	139	215	[1]	470	1,037	[1]
Gross Realized Gains	11	14		36	47
Gross Realized Losses	(3)	(11)		(25)	(24)
Net Realized Gains (Losses)	$ 8	$ 3		$ 11	$ 23

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