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Prepaid Expenses and Other Current Assets (Tables)
12 Months Ended
Dec. 31, 2017
Prepaid Expenses And Other Current Assets [Abstract]  
Schedule of prepaid expenses and other current assets
     
December 31,
 
     
2016
   
2017
 
Research and development tax credit receivable
      $ 1,730         $ 1,054    
Prepayments
        867           363    
Delaware tax receivable
                  25    
VAT receivable
        327           409    
Deposits
        132           132    
Other current assets
        41           81    
Prepaid expenses and other current assets
      $ 3,097         $ 2,064