Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 28	$ 27	$ 57	$ 54
Interest Cost	51	51	102	101
Expected Return on Plan Assets	(99)	(99)	(197)	(197)
Amortization of Prior Service Cost	(4)	(5)	(9)	(9)
Amortization of Actuarial Loss	25	40	49	79
Total Benefit Costs	1	14	2	28
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4	4	8	8
Interest Cost	16	14	32	29
Expected Return on Plan Assets	(9)	(7)	(17)	(15)
Amortization of Prior Service Cost	(2)	(4)	(5)	(7)
Amortization of Actuarial Loss	12	10	25	20
Total Benefit Costs	$ 21	$ 17	$ 43	$ 35