Available-For-Sale Securities (Proceeds From The Sales Of And The Net Realized Gains On Securities) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale and Maturity of Available-for-sale Securities	[1]	$ 711	$ 1,397	$ 1,448
Gross Realized Gains		53	97	177
Gross Realized Losses		(32)	(37)	(23)
Net Realized Gains (Losses)		21	60	154
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale and Maturity of Available-for-sale Securities	[2]	113	104	467
Gross Realized Gains		6	3	4
Gross Realized Losses		(5)	(2)	(3)
Net Realized Gains (Losses)		$ 1	$ 1	$ 1

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

[2] (A)Includes activity in accounts related to the liquidation of funds being transitioned to new managers.