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				9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		$ 45		$ 18		$ 31		$ 71
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sales		33	[1]	24	[1]	80	[2]	168	[2]
Gross Realized Gains		0		0		2		17
Gross Realized Losses		(1)		(1)		(3)		(5)
Net Realized Gains (Losses)		(1)		(1)		(1)	[3]	12	[3]
Unrealized Gain (Loss) on Securities	[4]	2				2
Other than Temporary Impairment Losses, Investments		0		0		0		0
Net Gains (Losses) on Trust Investments		1		(1)		1		12
Power [Member]
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		44		19		30		62
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sales	[1]	231		278		1,005		845
Gross Realized Gains		17		29		75		82
Gross Realized Losses		(7)		(5)		(29)		(14)
Net Realized Gains (Losses)		10		24		46	[5]	68	[5]
Unrealized Gain (Loss) on Securities	[6]	34				(16)
Other than Temporary Impairment Losses, Investments		0		(5)		0		(9)
Net Gains (Losses) on Trust Investments		$ 44		$ 19		$ 30		$ 59

[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).