Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)
$ in Thousands
Convertible preferred stock
Common stock
Treasury stock
Additional paid-in capital
Accumulated deficit
Total
Balance, beginning of year at Dec. 31, 2012 $ 150,456          
Balance, beginning of year (in shares) at Dec. 31, 2012 24,593,087          
Balance, end of year at Dec. 31, 2013 $ 150,456          
Balance, end of year (in shares) at Dec. 31, 2013 24,593,087          
Balance, beginning of year at Dec. 31, 2012   $ 3   $ 1,467 $ (109,110) $ (107,640)
Balance, beginning of year (in shares) at Dec. 31, 2012   276,854        
Changes in Stockholders' Equity            
Repurchased common shares     $ (260)     (260)
Repurchased common shares (in shares)     72,727      
Employee stock-based compensation expense       342   342
Stock option exercises       10   10
Stock option exercises (in shares)   3,025        
Net loss         (19,125) (19,125)
Balance, end of year at Dec. 31, 2013   $ 3 $ (260) 1,819 (128,235) (126,673)
Balance, end of year (in shares) at Dec. 31, 2013   279,879 72,727      
Changes in Temporary Equity            
Conversion of convertible preferred stock to common stock $ (150,456)          
Conversion of convertible preferred stock to common stock (in shares) (24,593,087)          
Changes in Stockholders' Equity            
Conversion of convertible preferred stock to common stock   $ 89   150,367   150,456
Conversion of convertible preferred stock to common stock (in shares)   8,942,925        
Proceeds from IPO, net of costs   $ 58   76,977   77,035
Proceeds from IPO, net of costs (in shares)   5,750,000        
Employee stock-based compensation expense       594   594
Stock option exercises       38   38
Stock option exercises (in shares)   12,900        
Net loss         (5,811) (5,811)
Balance, end of year at Dec. 31, 2014   $ 150 $ (260) 229,795 (134,046) 95,639
Balance, end of year (in shares) at Dec. 31, 2014   14,985,704 72,727      
Changes in Stockholders' Equity            
Proceeds from follow-on offering, net of costs   $ 30   61,367   61,397
Proceeds from follow-on offering, net of costs (in shares)   3,000,000        
Employee stock-based compensation expense       2,382   2,382
Stock option exercises       119   119
Stock option exercises (in shares)   36,189        
Employee stock purchase program (ESPP)       564   564
Employee stock purchase program (ESPP) (in shares)   44,250        
Net loss         (41,230) (41,230)
Balance, end of year at Dec. 31, 2015   $ 180 $ (260) $ 294,227 $ (175,276) $ 118,871
Balance, end of year (in shares) at Dec. 31, 2015   18,066,143 72,727