Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 27	$ 31
Interest Cost	50	59
Expected Return on Plan Assets	(98)	(103)
Amortization of Prior Service Cost	(4)	(5)
Amortization of Actuarial Loss	39	37
Total Benefit Costs	14	19
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4	5
Interest Cost	15	17
Expected Return on Plan Assets	(8)	(7)
Amortization of Prior Service Cost	(3)	(3)
Amortization of Actuarial Loss	10	10
Total Benefit Costs	$ 18	$ 22