Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Non-Operating Pension and OPEB (Credits) Costs	$ (19)	$ 0	$ (57)	$ (1)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	32	29	97	86
Interest Cost	52	51	156	153
Expected Return on Plan Assets	(111)	(98)	(331)	(295)
Amortization of Prior Service Cost	(4)	(5)	(13)	(14)
Amortization of Actuarial Loss	22	24	64	73
Non-Operating Pension and OPEB (Credits) Costs	(41)	(28)	(124)	(83)
Total Benefit Costs	(9)	1	(27)	3
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4	4	13	12
Interest Cost	16	15	49	47
Expected Return on Plan Assets	(9)	(8)	(30)	(25)
Amortization of Prior Service Cost	(1)	(3)	(1)	(8)
Amortization of Actuarial Loss	16	13	48	38
Non-Operating Pension and OPEB (Credits) Costs	22	17	66	52
Total Benefit Costs	$ 26	$ 21	$ 79	$ 64