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Postretirement benefits other than pensions - Schedule Of 1% Change in Health Care Cost Trend Rates (Details) - Other Postretirement Benefits Plan - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 30, 2018
Effect of 1% increase in health care cost trend rates    
Accumulated postretirement benefit obligation $ 4,431 $ 4,617
Dollar change $ 307 $ 287
Percent change 7.50% 6.60%
Effect of 1% decrease in health care cost trend rates    
Accumulated postretirement benefit obligation $ 3,858 $ 4,082
Dollar change $ (265) $ (248)
Percent change (6.40%) (5.70%)