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
Amounts recognized in Regulatory assets (see Note 8)
Regulatory Assets	$ 3,727	$ 2,863
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation, end of year	4,853	4,068
Change in accumulated postretirement benefit obligation
Benefit obligation, beginning of year	4,380	4,729
Service cost	83	94	82
Interest cost	212	192	204
Plan amendments	(7)	(3)
Actuarial (gain) loss	836	(400)
Benefits paid	(235)	(232)
Benefit obligation, end of year	5,269	4,380	4,729
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of year	3,720	3,223
Actual return on plan assets	301	445
Company contributions	195	284
Benefits paid	(235)	(232)
Plan assets at fair value, end of year	3,981	3,720	3,223
Funded status of the plans	(1,288)	(660)
Amount recorded as:
Current liabilities	(8)	(7)
Noncurrent liabilities	(1,280)	(653)
Pension and Other Postretirement Defined Benefit Plans, Liabilities	(1,288)	(660)
Amounts recognized in Accumulated other comprehensive loss, pre-tax
Net actuarial loss	194	174
Prior service (credit)	(1)	(1)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	193	173
Amounts recognized in Regulatory assets (see Note 8)
Net actuarial loss	2,285	1,654
Prior service credit	(1)	6
Regulatory Assets	$ 2,284	$ 1,660