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Trade and other receivables, net - Movements in allowance for trade receivables (Details) - Trade and other receivables - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance For Doubtful Trade Receivables [Roll Forward]      
Beginning balance $ 643 $ 481 $ 527
Charge of the year 758 387 152
Unused amounts reversed (133) (178) (27)
Used during the year (193) 0 (7)
Exchange differences (73) (47) (164)
Ending balance $ 1,002 $ 643 $ 481