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Prepaid Expenses and Other Current Assets (Tables)
6 Months Ended
Jun. 30, 2018
Prepaid Expenses And Other Current Assets [Abstract]  
Schedule of prepaid expenses and other current assets
    December 31,     June 30,  
    2017     2018  
Research and development tax credit receivable   $ 1,054     $ 1,693  
Prepayments and VAT receivable     772       998  
Other current assets     238       172  
    $ 2,064     $ 2,863