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Retirement Benefits (Benefit Assumptions) (Details)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year 7.25% 7.15%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 4.83% 4.82%
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 4.00% 4.02%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.33% 3.52%
Expected long term return on assets, tax exempt accounts 3.37% 3.26%
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.71% 3.40%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.76% 3.51%
Expected long term return on assets, tax exempt accounts 7.00% 6.63%
Expected long term return on assets, taxable accounts 4.75% 4.50%