Trust Investments (Proceeds From The Sales Of And The Net Realized Gains On Securities) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		$ (143)	$ 134	$ (6)
Power [Member]
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		(140)	125	(6)
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sale and Maturity of Debt Securities, Available-for-sale	[1]	1,398	2,137		$ 711
Gross Realized Gains		121	157		53
Gross Realized Losses		(51)	(23)		(32)
Net Realized Gains (Losses)	[2]	70	134		21
Unrealized Gain (Loss) on Securities	[3]	(209)
Other than Temporary Impairment Losses, Investments		0	(12)	(28)
Net Gains (Losses) on Trust Investments		(139)	122	(7)
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sale and Maturity of Debt Securities, Available-for-sale	[4]	103	182		113
Gross Realized Gains		2	17		6
Gross Realized Losses		(4)	(5)		(5)
Net Realized Gains (Losses)	[5]	(2)	12		$ 1
Unrealized Gain (Loss) on Securities	[6]	(2)
Net Gains (Losses) on Trust Investments		$ (4)	$ 12	$ 1

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

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

[3] Effective January 1, 2018, unrealized gains (losses) on equity securities are recorded in Net Income instead of Other Comprehensive Income (Loss).

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

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

[6] Effective January 1, 2018, unrealized gains (losses) on equity securities are recorded in Net Income instead of Other Comprehensive Income (Loss).