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Retirement Benefits (Benefit Assumptions) (Details)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year 7.15% 7.20%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 4.82% 4.99%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate 2024 2023
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 4.02% 3.27%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.52% 3.52%
Expected long term return on assets, tax exempt accounts 3.26% 3.50%
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 3.40% 2.34%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.51% 3.10%
Expected long term return on assets, tax exempt accounts 6.63% 7.00%
Expected long term return on assets, taxable accounts 4.50% 4.50%