Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Non-Operating Pension and OPEB Credits (Costs)	$ 19	$ 0
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	32	29
Interest Cost	52	51
Expected Return on Plan Assets	(110)	(98)
Amortization of Prior Service Cost	(4)	(5)
Amortization of Actuarial Loss	21	24
Non-Operating Pension and OPEB Credits (Costs)	(41)	(28)
Total Benefit Costs	(9)	1
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4	4
Interest Cost	16	16
Expected Return on Plan Assets	(10)	(8)
Amortization of Prior Service Cost	0	(3)
Amortization of Actuarial Loss	16	13
Non-Operating Pension and OPEB Credits (Costs)	22	18
Total Benefit Costs	$ 26	$ 22