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Retirement Benefits (Schedule of Weighted Average Assumptions) (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Pension Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items]      
Discount rate for benefit obligation 2.40% 3.00% 4.00%
Discount rate for net benefit costs 3.00% 4.00% 3.40%
Rate of compensation increase for benefit obligation 3.30% 3.30% 3.40%
Rate of compensation increase for net benefit costs 3.30% 3.40% 3.40%
Expected return on plan assets for net benefit costs 7.30% 7.40% 7.40%
Retiree Health Benefit Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items]      
Discount rate for benefit obligation 2.60% 3.30% 4.40%
Discount rate for net benefit costs 3.30% 4.40% 3.70%
Expected return on plan assets for net benefit costs 6.00% 6.00% 8.00%