Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Increase (Decrease) in Obligation, Pension Benefits	$ 23
Increase (Decrease) in Obligation, Other Postretirement Benefits	14
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax				$ (18)
Defined Benefit Plan Amounts Recognized In Regulatory Assets Before Tax	61
Defined Benefit Plan Amounts Recognized In Regulatory Liabilities Before Tax	$ 1
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets				7.00%
Discount rate				3.62%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets				6.60%
Discount rate				3.61%
Fitzpatrick [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (Decrease) in Obligation, Pension Benefits		$ 16	$ 38
Increase (Decrease) in Obligation, Other Postretirement Benefits		$ 17	$ 11