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Prepaid Expenses and Other Current Assets (Tables)
9 Months Ended
Sep. 30, 2017
Prepaid Expenses And Other Current Assets [Abstract]  
Schedule of prepaid expenses and other current assets
    December 31,     September 30,  
    2016     2017  
Research and development tax credit receivable   $ 1,730     $ 845  
Prepayments     867       365  
VAT receivable     327       376  
Other current assets     173       206  
    $ 3,097     $ 1,792