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Pension benefits - Weighted-Average Asset Allocation of Funds Related to Defined Benefit Plan (Details)
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocation of funds related to defined benefit plan (as percent) 100.00% 100.00%
Equity securities    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocation of funds related to defined benefit plan (as percent) 7.20% 13.60%
Debt securities    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocation of funds related to defined benefit plan (as percent) 68.20% 58.40%
Real estate    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocation of funds related to defined benefit plan (as percent) 13.60% 11.00%
Money market    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocation of funds related to defined benefit plan (as percent) 10.60% 16.50%
Other    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocation of funds related to defined benefit plan (as percent) 0.40% 0.50%