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Stockholders' Equity - Common Stock Warrants (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jun. 30, 2016
Feb. 29, 2016
Class of Warrant or Right [Line Items]          
Stock-based compensation expense $ 12,821 $ 5,061 $ 2,997    
Warrants to purchase common stock          
Class of Warrant or Right [Line Items]          
Warrants outstanding (in shares) 1,419,415 1,607,864      
Warrants exercised (in shares) 188,449 482,661      
Warrants exercised, shares issued (in shares) 177,854 441,307      
Warrants cancelled (in shares)   69,114      
Stock-based compensation expense $ 0 $ 0 $ 300    
Warrants to purchase common stock | Weighted Average          
Class of Warrant or Right [Line Items]          
Exercise price (in dollars per share) $ 1.73 $ 1.59      
Warrants to purchase common stock, issued February 2016          
Class of Warrant or Right [Line Items]          
Shares issued upon the exercise of warrants (in shares)         71,280
Exercise price (in dollars per share)       $ 1.41 $ 0.86
Warrants, term         5 years
Warrants to purchase common stock, issued June 2016          
Class of Warrant or Right [Line Items]          
Shares issued upon the exercise of warrants (in shares)       69,114  
Warrants, term       5 years