Pension, OPEB and Savings Plans (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 123	$ 130	$ 114
Interest Cost	218	208	204
Expected Return on Plan Assets	(408)	(441)	(394)
Amortization of Prior Service Cost	(18)	(18)	(18)
Amortization of Net Actuarial Gain (Loss)	96	85	97
Non-Operating Pension and Other Postretirement Plan (Credits) Costs	(112)	(166)	(111)
Net Periodic Benefit Cost	11	(36)	3
Total Benefit Costs, Including Effect of Regulatory Asset	11	(36)	3
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	10	18	17
Interest Cost	45	66	63
Expected Return on Plan Assets	(36)	(41)	(34)
Amortization of Prior Service Cost	(128)	(1)	(11)
Amortization of Net Actuarial Gain (Loss)	50	64	51
Non-Operating Pension and Other Postretirement Plan (Credits) Costs	(69)	88	69
Net Periodic Benefit Cost	(59)	106	86
Total Benefit Costs, Including Effect of Regulatory Asset	$ (59)	$ 106	$ 86