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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, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]    
Health care cost trend rate - ultimate 4.50% 4.50%
Year in which ultimate trend rate achieved 2024 2024
Effect on total of service and interest cost components, one percentage point increase $ 245  
Effect on total of service and interest cost components, one percentage point decrease (203)  
Effect on postretirement benefit obligations, one percentage point increase 3,751  
Effect on postretirement benefit obligations, one percentage point decrease $ (3,155)  
Minimum    
Defined Benefit Plan Disclosure [Line Items]    
Health care trend rate assumed for next year 7.10% 7.50%
Maximum    
Defined Benefit Plan Disclosure [Line Items]    
Health care trend rate assumed for next year 7.40% 8.10%