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 9 Months Ended
Sep. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Jul. 24, 2018
Dec. 31, 2017
Class of Stock [Line Items]              
Shares of common stock outstanding (in shares) 54,149,719     54,149,719     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
Cash proceeds from option exercises       $ 983 $ 366    
Cash proceeds from warrant exercises       314 3,127    
Shares authorized for issuance to employees (in shares)           9,975,000  
2018 ESPP              
Class of Stock [Line Items]              
Shares authorized for issuance to employees (in shares)           3,000,000  
Shares reserved for ESPP (in shares)           500,000  
2016 ESPP              
Class of Stock [Line Items]              
Shares remaining and available for grant under ESPP (in shares)           74,000  
2016 and 2014 Plans              
Class of Stock [Line Items]              
Shares authorized for issuance to employees (in shares)           6,975,000  
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 $ 100   $ 200 1,000 400    
Cash proceeds from warrant exercises $ 100   $ 1,800 $ 300 $ 3,100    
Common Shares | Options              
Class of Stock [Line Items]              
Shares of common stock issued upon exercises (in shares) 75,634   142,555 594,769 232,017    
Common Shares | Warrants              
Class of Stock [Line Items]              
Shares of common stock issued upon exercises (in shares) 66,029   721,385 156,888 1,364,878