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Retirement Benefits Plans (Assumptions Used to Determine Defined Benefit Plan and Postretirement Benefit Plans Expense (Benefit) (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Postretirement benefit plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 2.90% 4.00% 3.35%