Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Non-Operating Pension and OPEB (Credits) Costs	$ (33)	$ (19)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	29	32
Interest Cost	58	52
Expected Return on Plan Assets	(97)	(110)
Amortization of Prior Service Cost	(5)	(4)
Amortization of Actuarial Loss	27	21
Non-Operating Pension and OPEB (Credits) Costs	(17)	(41)
Total Benefit Costs	12	(9)
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2	4
Interest Cost	11	16
Expected Return on Plan Assets	(9)	(10)
Amortization of Prior Service Cost	(32)	0
Amortization of Actuarial Loss	13	16
Non-Operating Pension and OPEB (Credits) Costs	(17)	22
Total Benefit Costs	$ (15)	$ 26