Retirement Benefits and Trusteed Assets (OPEB - Other Changes in Plan Assets and APBO Recognized in Regulatory Assets and OCI) (Details) (Other Postretirement Benefit Plan [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Other Postretirement Benefit Plan [Member]
Other changes in plan assets and benefit obligations recognized in Regulatory assets and Other comprehensive income
Net actuarial (gain) loss	$ 144	$ (258)
Amortization of net actuarial loss	(14)	(47)
Prior service cost	0	(159)
Amortization of prior service cost	109	100
Total recognized in Regulatory assets and Other comprehensive income	239	(364)
Total recognized in net periodic pension cost, Regulatory assets and Other comprehensive income	153	(389)
Estimated amounts to be amortized from Regulatory assets and Accumulated other comprehensive income into net periodic benefit cost during next fiscal year
Net actuarial loss	31	15
Prior service cost	$ (94)	$ (109)