Retirement Benefits - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Increase (Decrease) in Obligation, Pension Benefits	$ 75
Increase (Decrease) in Obligation, Other Postretirement Benefits	36
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	(39)	$ (39)
Defined Benefit Plan Amounts Recognized In Regulatory Assets Before Tax	53
Defined Benefit Plan Amounts Recognized In Regulatory Liabilities Before Tax	$ (5)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets		7.00%
Discount rate		4.31%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets		6.67%
Discount rate		4.30%