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Retirement Benefits (Benefit Assumptions) (Details)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year 7.30% 7.25%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 4.82% 4.83%
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.40% 4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 3.05% 3.33%
Expected long term return on assets, tax exempt accounts 4.57% 3.37%
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.04% 2.71%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 2.94% 3.76%
Expected long term return on assets, tax exempt accounts 7.00% 7.00%
Expected long term return on assets, taxable accounts 4.75% 4.75%