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Retirement plans - Actuarial assumptions and sensitivity analysis (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Discount rates 2.30% 3.00%
Discount rate    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Percentage of reasonably possible decrease in actuarial assumption 1.00%  
Increase (decrease) in defined benefit obligation due to reasonably possible decrease in actuarial assumption $ 6.9