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Employee benefit plans - Weighted average assumptions (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Weighted-Average Assumptions used to calculate benefit obligations    
Discount rate (as a percent) 2.50% 3.25%
Rate of compensation increase (as a percent) 2.00% 2.00%
Weighted-Average Assumptions used to calculate benefit expense    
Discount rate (as a percent) 3.30% 4.00%
Rate of return on plan assets (as a percent) 5.50% 5.80%
Rate of compensation increase (as a percent) 2.00% 2.00%
Period over which corporate AA rated yield curve extrapolated to derive CIA/Natcan curve 10 years  
Maturity period of corporate bonds rated AA 10 years