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Commitments and contingencies (Details Textual) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of commitments and contingent liabilities [Abstract]      
Minimum operating lease payments recognised as expense $ 2,332 $ 2,605 $ 2,930
Operating lease income $ 275 $ 436 $ 661