Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 36	$ 33	$ 106	$ 90
Interest Cost	48	51	144	167
Expected Return on Plan Assets	(111)	(108)	(332)	(301)
Amortization of Prior Service Cost	(3)	(4)	(8)	(13)
Amortization of Actuarial Loss	23	21	69	75
Non-Operating Pension and OPEB (Credits) Costs	(43)	(40)	(127)	(72)
Total Benefit (Credits) Costs	(7)	(7)	(21)	18
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2	2	7	7
Interest Cost	8	12	25	34
Expected Return on Plan Assets	(10)	(9)	(29)	(27)
Amortization of Prior Service Cost	(32)	(32)	(96)	(96)
Amortization of Actuarial Loss	12	13	35	38
Non-Operating Pension and OPEB (Credits) Costs	(22)	(16)	(65)	(51)
Total Benefit (Credits) Costs	$ (20)	$ (14)	$ (58)	$ (44)