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Employee future benefits - Net defined benefit liability (asset) (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) $ 4,299 $ 4,914  
Pension Plan      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) 4,215 4,794 $ 4,971
Other post-retirement benefit plan      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset) $ 84 $ 120 $ 196