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Pension benefits - Assumptions Used in Calculation of Pension Obligations (Details)
 6 Months Ended 12 Months Ended
Dec. 31, 2018
Jul. 01, 2018
Dec. 31, 2020
Dec. 31, 2019
Retirement Benefits [Abstract]        
Rate of compensation increase at the end of year (as percent) 2.75% 2.50% 2.25% 2.25%
Discount rate at the end of year (as percent) 2.60% 2.40% 1.70% 2.30%
Prescribed pension index factor (as percent) 2.00% 2.00% 1.20% 2.00%
Expected return on plan assets for the year (as percent) 2.60% 2.40% 2.60% 2.60%
Employee turnover (as percent) 4.00% 4.00% 4.00% 4.00%
Expected increases in Social Security Base (as percent) 2.50% 2.25% 2.00% 2.50%