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Fair Value Measurements - Financial Instruments Measured at Fair Value by Hierarchy Level and Significant Assumptions Used for Warrants (Details) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Liabilities:    
Common stock warrants   $ 29
Recurring | Level 1    
Assets:    
Total assets at fair value $ 16,818 35,870
Recurring | Level 1 | Cash and cash equivalents    
Assets:    
Cash and cash equivalents $ 16,818 35,870
Recurring | Level 3    
Liabilities:    
Total liabilities at fair value   29
Recurring | Level 3 | Warrants | Common stock warrants    
Liabilities:    
Common stock warrants   $ 29
Recurring | Level 3 | Warrants | Common stock warrants | Option pricing model    
Significant assumptions used in preparing the option pricing model for valuing the Company's warrants    
Volatility (as a percent) 70.00% 70.00%
Risk free interest rate (as a percent) 2.59% 1.89%
Life of treasury bonds 1 year 3 months 2 years
Strike price (in dollars per share) $ 6.00 $ 6.00
Fair value of common stock (in dollars per share) $ 0.37 $ 2.69
Expected life (in years) 1 year 3 months 2 years