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, 2020	Dec. 31, 2019
Other changes in plan assets and benefit obligations recognized in Regulatory assets and Other comprehensive income (loss)
Net actuarial loss	$ 137	$ 156
Amortization of net actuarial loss	(193)	(133)
Amortization of prior service cost	(1)	(1)
Total recognized in Regulatory assets and Other comprehensive income (loss)	(57)	22
Total recognized in net periodic pension cost, Regulatory assets, and Other comprehensive income (loss)	$ 91	$ 134