Retirement Benefits and Trusteed Assets (Pension Plan - Other Changes in Plan Assets and Benefit Obligations recognized in Reg Assets and OCI) (Details) - Pension plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other changes in plan assets and benefit obligations recognized in Regulatory assets and Other comprehensive income (loss)
Net actuarial loss	$ 27	$ 197
Amortization of net actuarial loss	(176)	(164)
Prior service cost (credit)	(11)	7
Amortization of prior service cost	(1)	(1)
Total recognized in Regulatory assets and Other comprehensive income (loss)	(161)	39
Total recognized in net periodic pension cost, Regulatory assets, and Other comprehensive income (loss)	11	206
Estimated amounts to be amortized from Regulatory assets and Accumulated other comprehensive income (loss) into net periodic benefit cost during next fiscal year
Net actuarial loss	178	172
Prior service cost	$ 0	$ 1