Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 29	$ 27
Interest Cost	51	50
Expected Return on Plan Assets	(98)	(98)
Amortization of Prior Service Cost	(5)	(4)
Amortization of Actuarial Loss	24	39
Total Benefit Costs	1	14
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4	4
Interest Cost	16	15
Expected Return on Plan Assets	(8)	(8)
Amortization of Prior Service Cost	(3)	(3)
Amortization of Actuarial Loss	13	10
Total Benefit Costs	$ 22	$ 18