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Employee future benefits - Additional Information (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
plan
Disclosure of net defined benefit liability (asset) [line items]  
Estimate of contributions expected to be paid to plan for next annual reporting period | $ $ 500
Number of plans | plan 2
Actuarial assumption of medical cost trend rates  
Disclosure of net defined benefit liability (asset) [line items]  
Percentage of reasonably possible decrease in actuarial assumption 1.00%
Percentage of reasonably possible increase in actuarial assumption 1.00%