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Retirement Benefits (Net Periodic Benefit Costs Schedule) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Interest Cost $ 2 $ 3
Defined Benefit Plan, Expected Return (Loss) on Plan Assets 0 (1)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) 0 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 2 2
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Interest Cost 4 4
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (9) (8)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) 2 1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) $ (3) $ (3)