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Health care rate assumptions and cost trend rate (Details 7) - Overfunded Plan - Qualified Plan - Other Postretirement Benefits - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Health care trend rate assumed for next year 7.00%  
Health care cost trend rate - ultimate 4.50% 4.50%
Year in which ultimate trend rate achieved 2031 2024
Minimum    
Defined Benefit Plan Disclosure [Line Items]    
Health care trend rate assumed for next year   7.10%
Maximum    
Defined Benefit Plan Disclosure [Line Items]    
Health care trend rate assumed for next year   7.40%
Effect on total of service and interest cost components, one percentage point increase $ 182  
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components (154)  
Effect on postretirement benefit obligations, one percentage point increase 3,846  
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation $ (3,304)