Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 35	$ 29
Interest Cost	48	58
Expected Return on Plan Assets	(111)	(97)
Amortization of Prior Service Cost	(2)	(5)
Amortization of Actuarial Loss	23	27
Non-Operating Pension and OPEB (Credits) Costs	(42)	(17)
Total Benefit Costs	(7)	12
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2	2
Interest Cost	9	11
Expected Return on Plan Assets	(10)	(9)
Amortization of Prior Service Cost	(32)	(32)
Amortization of Actuarial Loss	12	13
Non-Operating Pension and OPEB (Credits) Costs	(21)	(17)
Total Benefit Costs	$ (19)	$ (15)