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, 2018	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized in Regulatory assets
Regulatory assets	$ 4,721	$ 3,778
Pension plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation, end of year	4,779	5,149
Change in projected benefit obligation
Projected/Accumulated postretirement benefit obligation, beginning of year	5,576	5,171
Service cost	99	92	$ 92
Interest cost	202	214	219
Plan amendments	0	(11)
Actuarial (gain) loss	(438)	391
Benefits paid	(315)	(281)
Projected/Accumulated postretirement benefit obligation, end of year	5,124	5,576	5,171
Change in plan assets
Plan assets at fair value, beginning of year	4,636	4,012
Actual return on plan assets	(233)	674
Company contributions	185	231
Benefits paid	(315)	(281)
Plan assets at fair value, end of year	4,273	4,636	$ 4,012
Funded status	(851)	(940)
Amount recorded as:
Current liabilities	(14)	(16)
Noncurrent liabilities	(837)	(924)
Defined benefit plans liabilities	(851)	(940)
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax
Net actuarial loss	152	163
Prior service cost	5	6
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax	157	169
Amounts recognized in Regulatory assets
Net actuarial loss	1,973	2,014
Prior service credit	(12)	(14)
Regulatory assets	$ 1,961	$ 2,000