Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity (Details)

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Stockholders' Equity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Jul. 24, 2018
Dec. 31, 2017
Class of Stock [Line Items]              
Shares of common stock outstanding (in shares) 54,008,056     54,008,056     45,017,062
Shares of preferred stock outstanding (in shares) 2,907     2,907     4,963
Par value of common stock (in dollars per share) $ 0.001     $ 0.001     $ 0.001
Par value of preferred stock (in dollars per share) $ 0.001     $ 0.001     $ 0.001
Cash proceeds from option exercises       $ 839 $ 178    
Cash proceeds from warrant exercises       182 1,287    
2018 ESPP | Subsequent Event              
Class of Stock [Line Items]              
Shares approved for issuance under ESPP (in shares)           3,000,000  
Shares reserved for ESPP (in shares)           500,000  
2016 ESPP | Subsequent Event              
Class of Stock [Line Items]              
Shares remaining and available for grant under ESPP (in shares)           74,000  
Preferred Stock              
Class of Stock [Line Items]              
Shares of preferred stock converted to common stock (in shares)   2,056          
Common Shares              
Class of Stock [Line Items]              
Shares of common stock issued upon conversion of preferred stock (in shares)   2,056,000          
Cash proceeds from option exercises $ 0   $ 200 800 200    
Cash proceeds from warrant exercises $ 100     $ 200 $ 1,300    
Common Shares | Options              
Class of Stock [Line Items]              
Shares of common stock issued upon exercises (in shares) 6,175   89,462 519,135 89,462.000    
Common Shares | Warrants              
Class of Stock [Line Items]              
Shares of common stock issued upon exercises (in shares) 27,697   692,620 90,859 692,620