Retirement Benefits - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Increase (Decrease) in Obligation, Pension Benefits	$ 92
Increase (Decrease) in Obligation, Other Postretirement Benefits	57
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	59
Defined Benefit Plan Amounts Recognized In Regulatory Assets Before Tax	57
Defined Benefit Plan Amounts Recognized In Regulatory Liabilities Before Tax	$ 4
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	7.00%
Discount rate	4.04%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	6.58%
Discount rate	4.04%
Fitzpatrick [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (Decrease) in Obligation, Pension Benefits	$ 38
Increase (Decrease) in Obligation, Other Postretirement Benefits	$ 11