Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 29	$ 28	$ 86	$ 82
Interest Cost	51	50	153	151
Expected Return on Plan Assets	(98)	(98)	(295)	(295)
Amortization of Prior Service Cost	(5)	(5)	(14)	(14)
Amortization of Actuarial Loss	24	39	73	118
Total Benefit Costs	1	14	3	42
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4	5	12	13
Interest Cost	15	15	47	44
Expected Return on Plan Assets	(8)	(8)	(25)	(23)
Amortization of Prior Service Cost	(3)	(4)	(8)	(11)
Amortization of Actuarial Loss	13	10	38	30
Total Benefit Costs	$ 21	$ 18	$ 64	$ 53