Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 38	$ 35	$ 76	$ 70
Interest Cost	35	48	70	96
Expected Return on Plan Assets	(119)	(110)	(238)	(221)
Amortization of Prior Service Cost	0	(3)	0	(5)
Amortization of Actuarial Loss	25	23	51	46
Non-Operating Pension and OPEB (Credits) Costs	(59)	(42)	(117)	(84)
Total Benefit (Credits) Costs	(21)	(7)	(41)	(14)
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2	3	4	5
Interest Cost	6	8	11	17
Expected Return on Plan Assets	(11)	(9)	(21)	(19)
Amortization of Prior Service Cost	(32)	(32)	(64)	(64)
Amortization of Actuarial Loss	11	11	22	23
Non-Operating Pension and OPEB (Credits) Costs	(26)	(22)	(52)	(43)
Total Benefit (Credits) Costs	$ (24)	$ (19)	$ (48)	$ (38)