Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Non-Operating Pension and OPEB (Credits) Costs	$ (19)	$ (1)	$ (38)	$ (1)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	33	28	65	57
Interest Cost	52	51	104	102
Expected Return on Plan Assets	(110)	(99)	(220)	(197)
Amortization of Prior Service Cost	(5)	(4)	(9)	(9)
Amortization of Actuarial Loss	21	25	42	49
Non-Operating Pension and OPEB (Credits) Costs	(42)	(27)	(83)	(55)
Total Benefit Costs	(9)	1	(18)	2
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	5	4	9	8
Interest Cost	17	16	33	32
Expected Return on Plan Assets	(11)	(9)	(21)	(17)
Amortization of Prior Service Cost	0	(2)	0	(5)
Amortization of Actuarial Loss	16	12	32	25
Non-Operating Pension and OPEB (Credits) Costs	22	17	44	35
Total Benefit Costs	$ 27	$ 21	$ 53	$ 43