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Prepaid Expenses and Other Current Assets (Tables)
3 Months Ended
Mar. 31, 2017
Prepaid Expenses And Other Current Assets [Abstract]  
Schedule of prepaid expenses and other current assets
    December 31,     March 31,  
    2016     2017  
Research and development tax credit receivable   $ 1,730     $ 2,053  
Prepayments     867       705  
Accounts receivable     10       879  
Other taxes receivable     327       399  
Deposits     132       132  
Other current assets     31       137  
    $ 3,097     $ 4,305