Retirement Benefits and Trusteed Assets (Pension Plan - Reconciliation of Obligations, Assets and Funded Status of Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in projected benefit obligation
Plan amendments	$ (2)	$ 0
Amounts recognized in Regulatory assets (liabilities) (see Note 7 - Regulatory Matters)
Regulatory Assets	2,959	2,288
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation, end of year	3,712	3,111
Change in projected benefit obligation
Projected benefit obligation, beginning of year	3,341	3,585
Service cost	64	73	64
Interest cost	162	146	155
Actuarial (gain) loss	634	(286)
Benefits paid	(181)	(177)
Projected benefit obligation, end of year	4,018	3,341	3,585
Change in plan assets
Plan assets at fair value, beginning of year	2,632	2,211
Actual return on plan assets	212	316
Company contributions	149	282
Benefits paid	(181)	(177)
Plan assets at fair value, end of year	2,812	2,632	2,211
Funded status of the plan	(1,206)	(709)
Amount recorded as:
Current liabilities	(6)	(4)
Noncurrent liabilities	(1,200)	(705)
Pension and Other Postretirement Defined Benefit Plans, Liabilities	(1,206)	(709)
Amounts recognized in Regulatory assets (liabilities) (see Note 7 - Regulatory Matters)
Net actuarial loss	1,738	1,248
Prior service cost	5	9
Regulatory Assets	$ 1,743	$ 1,257