Pension, OPEB and Savings Plans (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 114	$ 109	$ 123
Interest Cost	204	202	234
Expected Return on Plan Assets	(394)	(394)	(414)
Amortization of Prior Service Cost	(18)	(19)	(19)
Amortization of Actuarial Loss	97	158	150
Net Periodic Benefit Cost	3	56	74
Total Benefit Costs, Including Effect of Regulatory Asset	3	56	74
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	17	17	22
Interest Cost	63	59	67
Expected Return on Plan Assets	(34)	(31)	(31)
Amortization of Prior Service Cost	(11)	(14)	(14)
Amortization of Actuarial Loss	51	40	43
Net Periodic Benefit Cost	86	71	87
Total Benefit Costs, Including Effect of Regulatory Asset	$ 86	$ 71	$ 87