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Accounts receivable, net - Schedule of Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]      
Balance at beginning of the year $ 119 $ 126 $ 131
Additions 27 0 4
Write-off (122) 0 0
Exchange difference 7 (7) (9)
Balance at end of the year $ 31 $ 119 $ 126