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