Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)
$ in Thousands
Total
Redeemable Convertible Preferred Stock [Member]
Redeemable Convertible Preferred Stock [Member]
Series E Preferred Stock [Member]
Common Stock [Member]
Additional Paid-in Capital [Member]
Accumulated Deficit [Member]
Accumulated Other Comprehensive Loss [Member]
Beginning balance at Dec. 31, 2013 $ (68,574)     $ 1 $ 332 $ (68,907)  
Issuance of redeemable convertible preferred stock, value     $ 442        
Beginning balance, shares at Dec. 31, 2013       729,391      
Temporary equity, beginning balance at Dec. 31, 2013   $ 103,797          
Temporary equity, beginning balance, shares at Dec. 31, 2013   94,483,404          
Accretion of redeemable convertible preferred stock to redemption value (92) $ 92     (92)    
Conversion of redeemable convertible preferred stock to common stock, value 104,331 $ (104,331)   $ 15 104,316    
Conversion of redeemable convertible preferred stock to common stock, shares   (94,687,505)   15,077,621      
Issuance of common stock 100,164     $ 7 100,157    
Issuance of common stock, shares     204,101 6,900,000      
Issuance of common stock upon exercise of stock options, value 3       3    
Issuance of common stock upon exercise of stock options, shares       398      
Issuance of common stock in lieu of milestone payment 3,569       3,569    
Issuance of common stock in lieu of milestone payment, shares       171,750      
Stock-based compensation expense 1,553       1,553    
Unrealized gain (loss) on marketable securities (35)           $ (35)
Net loss (36,478)         (36,478)  
Ending balance at Dec. 31, 2014 104,441     $ 23 209,838 (105,385) (35)
Ending balance, shares at Dec. 31, 2014       22,879,160      
Issuance of common stock upon exercise of stock options and employee stock purchase plan 974       974    
Issuance of common stock upon exercise of stock options and employee stock purchase plan, shares       418,241      
Issuance of common stock 129,571     $ 4 129,567    
Issuance of common stock, shares       3,942,200      
Issuance of restricted stock units 0 $ 0   $ 0 0 0 0
Issuance of restricted stock units, shares       2,902      
Stock-based compensation expense 8,582       8,582    
Unrealized gain (loss) on marketable securities (172)           (172)
Net loss (74,286)         (74,286)  
Ending balance at Dec. 31, 2015 169,110     $ 27 348,961 (179,671) (207)
Ending balance, shares at Dec. 31, 2015       27,242,503      
Issuance of common stock upon exercise of stock options and employee stock purchase plan 220       220    
Issuance of common stock upon exercise of stock options and employee stock purchase plan, shares       72,663      
Issuance of common stock 35       35    
Issuance of common stock, shares       5,564      
Issuance of restricted stock units $ 0 $ 0   $ 0 0 0 0
Issuance of common stock upon exercise of stock options, shares 35,000            
Issuance of restricted stock units, shares       11,821      
Stock-based compensation expense $ 10,113       10,113    
Unrealized gain (loss) on marketable securities 127           127
Net loss (57,878)         (57,878)  
Ending balance at Dec. 31, 2016 $ 121,727     $ 27 $ 359,329 $ (237,549) $ (80)
Ending balance, shares at Dec. 31, 2016       27,332,551