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, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized in Regulatory assets
Regulatory assets	$ 3,778	$ 3,913
Pension plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation, end of year	5,149	4,753
Change in projected benefit obligation
Projected/Accumulated postretirement benefit obligation, beginning of year	5,171	4,971
Service cost	92	92	$ 100
Interest cost	214	219	210
Plan amendments	(11)	7
Actuarial loss	391	141
Benefits paid	(281)	(259)
Projected/Accumulated postretirement benefit obligation, end of year	5,576	5,171	4,971
Change in plan assets
Plan assets at fair value, beginning of year	4,012	3,832
Actual return on plan assets	674	253
Company contributions	231	186
Benefits paid	(281)	(259)
Plan assets at fair value, end of year	4,636	4,012	$ 3,832
Funded status	(940)	(1,159)
Amount recorded as:
Current liabilities	(16)	(7)
Noncurrent liabilities	(924)	(1,152)
Defined benefit plans liabilities	(940)	(1,159)
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax
Net actuarial loss	163	163
Prior service cost	6	8
Amounts recognized in Accumulated other comprehensive income (loss), pre-tax	169	171
Amounts recognized in Regulatory assets
Net actuarial loss	2,014	2,163
Prior service credit	(14)	(4)
Regulatory assets	$ 2,000	$ 2,159