Pension And OPEB (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 26	$ 22	$ 76	$ 69
Interest Cost	56	56	167	172
Expected Return on Plan Assets	(76)	(85)	(229)	(248)
Amortization of Net Transition Obligation	0	0	0	0
Amortization of Prior Service Cost	(5)	(4)	(14)	(6)
Amortization of Actuarial Loss	41	29	125	89
Net Periodic Benefit Cost	42	18	125	76
Special Termination Benefits	1	0	1	0
Effect of Regulatory Asset	0	0	0	0
Total Benefit Costs, Including Effect of Regulatory Asset	43	18	126	76
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	5	3	13	10
Interest Cost	17	15	49	45
Expected Return on Plan Assets	(4)	(5)	(13)	(13)
Amortization of Net Transition Obligation	1	1	2	4
Amortization of Prior Service Cost	(4)	(4)	(11)	(10)
Amortization of Actuarial Loss	7	4	23	11
Net Periodic Benefit Cost	22	14	63	47
Special Termination Benefits	0	0	0	0
Effect of Regulatory Asset	4	5	14	15
Total Benefit Costs, Including Effect of Regulatory Asset	$ 26	$ 19	$ 77	$ 62