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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
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Liabilities:    
Common stock warrants $ 406 $ 296
Significant assumptions used in preparing the option pricing model for valuing the Company's warrants    
Fair value of stock (in dollars per share) $ 9.76  
Recurring | Level 1    
Assets:    
Total assets at fair value $ 34,324 40,135
Recurring | Level 1 | Cash equivalents    
Assets:    
Cash and cash equivalents 34,324 40,135
Recurring | Level 3    
Liabilities:    
Total liabilities at fair value 406 296
Recurring | Level 3 | Warrants | Common stock warrants    
Liabilities:    
Common stock warrants $ 406 $ 296
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) 75.00% 104.80%
Risk free interest rate (as a percent) 1.54% 1.68%
Life of treasury bonds 4 years 5 years
Strike price (in dollars per share) $ 6.00 $ 6.00
Fair value of stock (in dollars per share) $ 9.76 $ 6.14
Expected life 4 years 5 years