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Financial - risk management objectives and policies (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of financial risk management objectives and policies [Line Items]      
Increase in exchange rate basis points 10.00% 10.00% 10.00%
Decrease in exchange rate basis points (10.00%) (10.00%) (10.00%)
Exchange rate increased effect on income $ 2,474 $ (924) $ 6,233
Exchange rate decreased effect on income $ (2,459) $ 926 $ (7,618)