Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Jun. 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