Retirement Benefits and Trusteed Assets (OPEB - Other Changes in Plan Assets and APBO Recognized in Regulatory Assets and OCI) (Details) (Other Postretirement Benefit Plan, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) loss	$ 192	$ (353)
Amortization of net actuarial loss	(20)	(64)
Prior service credit	0	(218)
Amortization of prior service credit	144	131
Total recognized Regulatory assets and Other comprehensive income	316	(504)
Total recognized in net periodic pension cost, Regulatory assets and Other comprehensive income	193	(546)
Future amortization of net actuarial loss into net periodic benefit cost	43	21
Future amortization of prior service credit into net periodic benefit cost	$ (126)	$ (144)