Pension, OPEB and Savings Plans (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 123	$ 104	$ 116
Interest Cost	234	234	215
Expected Return on Plan Assets	(414)	(399)	(348)
Amortization of Prior Service Cost	(19)	(18)	(19)
Amortization of Actuarial Loss	150	56	188
Net Periodic Benefit Cost	74	(23)	152
Total Benefit Costs, Including Effect of Regulatory Asset	74	(23)	152
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	22	18	21
Interest Cost	67	69	63
Expected Return on Plan Assets	(31)	(26)	(21)
Amortization of Prior Service Cost	(14)	(14)	(14)
Amortization of Actuarial Loss	43	23	42
Net Periodic Benefit Cost	87	70	91
Total Benefit Costs, Including Effect of Regulatory Asset	$ 87	$ 70	$ 91