Pension, OPEB and Savings Plans (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Non-Operating Pension and Other Postretirement Plan (Credits) Costs	$ (76)	$ 0	$ 22
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	130	114	109
Interest Cost	208	204	202
Expected Return on Plan Assets	(441)	(394)	(394)
Amortization of Prior Service Cost	(18)	(18)	(19)
Amortization of Actuarial Loss	85	97	158
Non-Operating Pension and Other Postretirement Plan (Credits) Costs	(166)	(111)	(53)
Net Periodic Benefit Cost	(36)	3	56
Total Benefit Costs, Including Effect of Regulatory Asset	(36)	3	56
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	18	17	17
Interest Cost	66	63	59
Expected Return on Plan Assets	(41)	(34)	(31)
Amortization of Prior Service Cost	(1)	(11)	(14)
Amortization of Actuarial Loss	64	51	40
Non-Operating Pension and Other Postretirement Plan (Credits) Costs	88	69	54
Net Periodic Benefit Cost	106	86	71
Total Benefit Costs, Including Effect of Regulatory Asset	$ 106	$ 86	$ 71