Retirement Benefits and Trusteed Assets (Pension Plan - Reconciliation of Obligations, Assets and Funded Status of Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amounts recognized in Regulatory assets
Regulatory assets	$ 4,176	$ 4,721
Pension plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation, end of year	5,387	4,779
Change in projected benefit obligation
Projected/Accumulated postretirement benefit obligation, beginning of year	5,124	5,576
Service cost	84	99	$ 92
Interest cost	219	202	214
Actuarial (gain) loss	719	(438)
Benefits paid	(336)	(315)
Projected/Accumulated postretirement benefit obligation, end of year	5,810	5,124	5,576
Change in plan assets
Plan assets at fair value, beginning of year	4,273	4,636
Actual return on plan assets	888	(233)
Company contributions	168	185
Benefits paid	(336)	(315)
Plan assets at fair value, end of year	4,993	4,273	$ 4,636
Funded status	(817)	(851)
Amount recorded as:
Current liabilities	(9)	(14)
Noncurrent liabilities	(808)	(837)
Defined benefit plans liabilities	(817)	(851)
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax
Net actuarial loss	153	152
Prior service cost	4	5
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax	157	157
Amounts recognized in Regulatory assets
Net actuarial loss	1,995	1,973
Prior service credit	(12)	(12)
Regulatory assets	$ 1,983	$ 1,961