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Retirement Benefits (Benefit Assumptions) (Details)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year 7.20% 6.73%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 4.99% 4.96%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate 2023 2021
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 3.27% 3.65%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.52% 3.60%
Expected long term return on assets, tax exempt accounts 3.50% 3.50%
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.34% 2.34%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.10% 3.06%
Expected long term return on assets, tax exempt accounts 7.00% 7.00%
Expected long term return on assets, taxable accounts 4.50% 4.50%