Pension, OPEB and Savings Plans (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 141	$ 123	$ 130
Interest Cost	192	218	208
Expected Return on Plan Assets	(443)	(408)	(441)
Amortization of Prior Service Cost	(10)	(18)	(18)
Amortization of Net Actuarial Gain (Loss)	92	96	85
Non-Operating Pension and Other Postretirement Plan (Credits) Costs	(169)	(112)	(166)
Net Periodic Benefit Cost	(28)	11	(36)
Total Benefit Costs, Including Effect of Regulatory Asset	(28)	11	(36)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	9	10	18
Interest Cost	34	45	66
Expected Return on Plan Assets	(39)	(36)	(41)
Amortization of Prior Service Cost	(128)	(128)	(1)
Amortization of Net Actuarial Gain (Loss)	47	50	64
Non-Operating Pension and Other Postretirement Plan (Credits) Costs	(86)	(69)	88
Net Periodic Benefit Cost	(77)	(59)	106
Total Benefit Costs, Including Effect of Regulatory Asset	$ (77)	$ (59)	$ 106