New And Significant Accounting Policies (Components Of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 119	$ 99
Interest cost	168	173
Expected return on plan assets	(218)	(202)
Amortization of prior service cost	4	5
Amortization of net actuarial loss	3	0
Net periodic benefit cost	76	75
Less: transfer to regulatory account	9	9
Total	85	84
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	13	11
Interest cost	18	19
Expected return on plan assets	(28)	(26)
Amortization of prior service cost	5	6
Amortization of net actuarial loss	1	0
Net periodic benefit cost	9	10
Less: transfer to regulatory account	0	0
Total	$ 9	$ 10