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Pension and Other Postretirement Benefits - Healthcare cost trend rates (Details) - Postretirement Benefits Medical and Prescription Drug
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Assumed healthcare cost trend rates used to determine benefit obligations and net cost:      
Rate that the cost trend rate is assumed to decline (the ultimate trend rate) 4.50% 4.50% 4.50%
Minimum      
Assumed healthcare cost trend rates used to determine benefit obligations and net cost:      
Healthcare cost trend rate assumed for next year 6.03% 6.25% 5.50%
Maximum      
Assumed healthcare cost trend rates used to determine benefit obligations and net cost:      
Healthcare cost trend rate assumed for next year 8.44% 9.00% 8.50%