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, 2017	Dec. 31, 2016	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]	$ 2,137	$ 711	$ 1,397
Gross Realized Gains		157	53	97
Gross Realized Losses		(23)	(32)	(37)
Net Realized Gains (Losses)		134	21	60
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale and Maturity of Available-for-sale Securities	[2]	182	113	104
Gross Realized Gains		17	6	3
Gross Realized Losses		(5)	(5)	(2)
Net Realized Gains (Losses)	[3]	$ 12	$ 1	$ 1

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

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

[3] The cost of these securities was determined on the basis of specific identification.