Retirement Benefits and Trusteed Assets (OPEB - 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 (liabilities) (see Note 8)
Regulatory Liabilities	$ (820)	$ (1,164)
Other Postretirement Benefit Plan
Change in accumulated postretirement benefit obligation
Benefit obligation, beginning of year	1,878	2,315
Service cost	34	47	68
Interest cost	89	88	120
Plan amendments	0	(218)
Actuarial (gain) loss	131	(267)
Medicare Part D subsidy	0	1
Benefits paid	(88)	(88)
Benefit obligation, end of year	2,044	1,878	2,315
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets	1,528	1,527	1,153
Actual return on plan assets	62	196
Company contributions	24	264
Benefits paid	(85)	(86)
Funded status of the plans	(516)	(351)
Amount recorded as:
Current liabilities	(1)	(1)
Noncurrent liabilities	(515)	(350)
Pension and Other Postretirement Defined Benefit Plans, Liabilities	(516)	(351)
Amounts recognized in Accumulated other comprehensive loss, pre-tax
Net actuarial loss	34	29
Prior service (credit)	(5)	(10)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	29	19
Amounts recognized in Regulatory assets (liabilities) (see Note 8)
Net actuarial loss	488	321
Prior service credit	(254)	(393)
Regulatory Liabilities	$ 234	$ (72)