Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 29	$ 25
Interest Cost	54	56
Expected Return on Plan Assets	(87)	(76)
Amortization of Net Transition Obligation	0	0
Amortization of Prior Service Cost	(5)	(5)
Amortization of Actuarial Loss	47	42
Net Periodic Benefit Cost	38	42
Effect of Regulatory Asset	0	0
Total Benefit Costs, Including Effect of Regulatory Asset	38	42
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	5	4
Interest Cost	16	16
Expected Return on Plan Assets	(5)	(4)
Amortization of Net Transition Obligation	0	1
Amortization of Prior Service Cost	(4)	(4)
Amortization of Actuarial Loss	11	8
Net Periodic Benefit Cost	23	21
Effect of Regulatory Asset	0	5
Total Benefit Costs, Including Effect of Regulatory Asset	$ 23	$ 26