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Financial Instruments and Fair Value Measurements - Schedule of Change in the Fair Value of the Warrant Liability (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Fair Value Disclosures [Abstract]    
Beginning balance $ 313 $ 577
Net exercise of warrants (133) (211)
Expiration of warrants, included in change in fair value of warrant liability   (87)
Change in fair value of warrant liability 609 46
Conversion of Series A preferred stock warrants to common stock warrants upon the closing of the IPO $ (789)  
Ending balance   $ 325