Retirement Benefits and Trusteed Assets (Pension Plan - Reconciliation of Obligations, Assets and Funded Status of Plans) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amounts recognized in Regulatory assets
Regulatory assets	$ 4,257	$ 4,257	$ 4,176
Pension plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation, end of year	5,843	5,843	5,387
Change in projected benefit obligation
Projected/Accumulated postretirement benefit obligation, beginning of year		5,810	5,124
Service cost		99	84	$ 99
Interest cost		186	219	202
Actuarial loss		619	719
Special termination benefits		3	0
Benefits paid		(353)	(336)
Settlements	(60)	(60)	0
Projected/Accumulated postretirement benefit obligation, end of year	6,304	6,304	5,810	5,124
Change in plan assets
Plan assets at fair value, beginning of year		4,993	4,273
Actual return on plan assets		815	888
Company contributions		102	168
Benefits paid		(353)	(336)
Settlements	(60)	(60)	0
Plan assets at fair value, end of year	5,497	5,497	4,993	$ 4,273
Funded status	(807)	(807)	(817)
Amount recorded as:
Current liabilities	(10)	(10)	(9)
Noncurrent liabilities	(797)	(797)	(808)
Defined benefit plans liabilities	(807)	(807)	(817)
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax
Net actuarial loss	142	142	153
Prior service cost	3	3	4
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax	145	145	157
Amounts recognized in Regulatory assets
Net actuarial loss	1,949	1,949	1,995
Prior service credit	(11)	(11)	(12)
Regulatory assets	$ 1,938	$ 1,938	$ 1,983