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
Change in projected benefit obligation
Plan amendments	$ (2)	$ 0
Amounts recognized in Regulatory assets (liabilities) (see Note 7 - Regulatory Matters)
Regulatory Liabilities	(395)	(664)
Other Postretirement Benefit Plan [Member]
Change in projected benefit obligation
Projected benefit obligation, beginning of year	1,430	1,752
Service cost	26	35	51
Interest cost	68	67	91
Plan amendments	0	(159)
Actuarial (gain) loss	100	(200)
Medicare Part D subsidy	0	1
Benefits paid	(66)	(66)
Projected benefit obligation, end of year	1,558	1,430	1,752
Change in plan assets
Plan assets at fair value, beginning of year	1,061	756
Actual return on plan assets	41	131
Company contributions	0	239
Benefits paid	(64)	(65)
Plan assets at fair value, end of year	1,038	1,061	756
Funded status of the plan	(520)	(369)
Amount recorded as:
Noncurrent liabilities	(520)	(369)
Amounts recognized in Regulatory assets (liabilities) (see Note 7 - Regulatory Matters)
Net actuarial loss	385	255
Prior service cost	(194)	(303)
Regulatory Liabilities	$ 191	$ (48)