Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 38	$ 35
Interest Cost	35	48
Expected Return on Plan Assets	(119)	(111)
Amortization of Prior Service Cost	0	(2)
Amortization of Actuarial Loss	26	23
Non-Operating Pension and OPEB (Credits) Costs	(58)	(42)
Total Benefit (Credits) Costs	(20)	(7)
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2	2
Interest Cost	5	9
Expected Return on Plan Assets	(10)	(10)
Amortization of Prior Service Cost	(32)	(32)
Amortization of Actuarial Loss	11	12
Non-Operating Pension and OPEB (Credits) Costs	(26)	(21)
Total Benefit (Credits) Costs	$ (24)	$ (19)