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Pension Plan and Other Postretirement Benefits - Healthcare Cost-trend Rates (Details) - Other Postretirement Benefits Plan
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Healthcare Cost-Trend Rate Assumed for Next Year 6.44% 6.72%
Rate to Which the Cost-Trend Rate is Assumed to Decline 4.50% 4.50%
Year the Rate Reaches the Ultimate Trend Rate 2038 2038