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Employee benefit plans - Schedule of Weighted-Average Assumptions Used to Determine Benefit Obligations and Related Expenses (Details) - Pension Benefits
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure      
Discount rate, Obligation 2.25% 3.25% 4.00%
Discount rate, Expense 3.25% 4.00% 3.75%
Expected long-term return on plan assets 7.50% 7.50% 7.50%