Trust Investments (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				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		$ 8		$ 25		$ (14)		$ 53
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sales		22	[1]	93	[1]	47	[2]	144	[2]
Gross Realized Gains		0		2		2		17
Gross Realized Losses		0		(1)		(2)		(4)
Net Realized Gains (Losses)		0		1		0	[3]	13	[3]
Unrealized Gain (Loss) on Securities	[4]	0				0
Other than Temporary Impairment Losses, Investments		0		0		0		0
Net Gains (Losses) on Trust Investments		0		1		0		13
Power [Member]
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		8		24		(14)		43
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sales	[1]	402		320		774		567
Gross Realized Gains		34		32		58		53
Gross Realized Losses		(10)		(5)		(22)		(9)
Net Realized Gains (Losses)		24		27		36	[5]	44	[5]
Unrealized Gain (Loss) on Securities	[6]	(16)				(50)
Other than Temporary Impairment Losses, Investments		0		(3)		0		(4)
Net Gains (Losses) on Trust Investments		$ 8		$ 24		$ (14)		$ 40

[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.

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

[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).