New And Significant Accounting Policies (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 115		$ 99
Interest cost	156		164
Expected return on plan assets	(162)		(149)
Amortization of transition obligation	0		0
Amortization of prior service cost	5		5
Amortization of unrecognized loss	27		31
Net periodic benefit cost	141		150
Less: transfer to regulatory account	(57)	[1]	(75)	[1]
Total	84		75
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	13		12
Interest cost	19		21
Expected return on plan assets	(20)		(19)
Amortization of transition obligation	0		6
Amortization of prior service cost	6		6
Amortization of unrecognized loss	1		1
Net periodic benefit cost	19		27
Less: transfer to regulatory account	0	[1]	0	[1]
Total	$ 19		$ 27

[1] The utility recorded these amounts to a regulatory account since they are probable of recovery from customers in future rates.