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Employee future benefits - Net defined benefit liability (asset) (Details) - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) $ 1,894 $ 3,941  
Pension Plan      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) 1,814 3,856 $ 4,308
Other post-retirement benefit plan      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) $ 80 $ 85 $ 88