Pension, OPEB and Savings Plans (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 109	$ 123	$ 104
Interest Cost	202	234	234
Expected Return on Plan Assets	(394)	(414)	(399)
Amortization of Prior Service Cost	(19)	(19)	(18)
Amortization of Actuarial Loss	158	150	56
Net Periodic Benefit Cost	56	74	(23)
Total Benefit Costs, Including Effect of Regulatory Asset	56	74	(23)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	17	22	18
Interest Cost	59	67	69
Expected Return on Plan Assets	(31)	(31)	(26)
Amortization of Prior Service Cost	(14)	(14)	(14)
Amortization of Actuarial Loss	40	43	23
Net Periodic Benefit Cost	71	87	70
Total Benefit Costs, Including Effect of Regulatory Asset	$ 71	$ 87	$ 70