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Employee benefits (Details 4) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Employee benefits [Abstract]      
Service cost or benefits received during the year $ 0 $ 0 $ 0
Interest cost in benefit obligation 280 336 319
Actual return in plan assets 4,642 741 133
Amortization of prior year losses 326 242 160
Net gain during the year 3,500 (854) (568)
Net periodic pension expense $ (31) $ (33) $ (159)