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Stockholders' Equity - Common stock (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Oct. 23, 2020
Sep. 30, 2020
Jun. 16, 2020
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Class of Stock [Line Items]                  
Sale of common stock, net       $ 6,354,000 $ 5,094,000 $ 7,078,000      
Proceeds from sale of common stock, net             $ 18,665,000 $ 0  
Common Stock [Member]                  
Class of Stock [Line Items]                  
Sale of common stock, net       $ 3,000 $ 2,000 $ 2,000      
Common stock issued (in shares)       3,272,500 1,907,000 2,534,879      
Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member]                  
Class of Stock [Line Items]                  
Sale of common stock, net               $ 25,000,000.0  
Period exercisable from the date of issuance               36 months  
Canaccord Genuity LLC [Member] | Common Stock [Member]                  
Class of Stock [Line Items]                  
Common stock issued (in shares)             2,179,193   1,616,331
Proceeds from sale of common stock, net $ 10,000,000           $ 6,300,000   $ 3,200,000
Remaining availability under financing facility 0                
2020 Repurchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member]                  
Class of Stock [Line Items]                  
Sale of common stock, net   $ 25,000,000.0              
Common stock issued (in shares)             5,535,186   353,113
Period exercisable from the date of issuance   36 months              
Closing price per share     $ 1.05            
Proceeds from sale of common stock, net             $ 12,300,000   $ 700,000
2020 Repurchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member] | Commitment Fee                  
Class of Stock [Line Items]                  
Common stock issued (in shares)                 180,701
2020 Repurchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member] | Minimum [Member]                  
Class of Stock [Line Items]                  
Percentage issuance of common stock     19.99%            
2020 Repurchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member] | Minimum [Member] | Single Regular Purchase [Member]                  
Class of Stock [Line Items]                  
Sale of common stock, net   $ 500,000              
Common stock issued (in shares)   50,000              
2020 Repurchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member] | Maximum [Member]                  
Class of Stock [Line Items]                  
Common stock issued (in shares)     23,800,000            
2020 Repurchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member] | Maximum [Member] | Single Regular Purchase [Member]                  
Class of Stock [Line Items]                  
Common stock issued (in shares)   100,000              
Closing price per share   $ 0.25           $ 0.25  
Sales Agreement | Canaccord Genuity LLC [Member] | Common Stock [Member] | At The Market Offering Program                  
Class of Stock [Line Items]                  
Remaining availability under financing facility $ 0                
Distribution Agreement [Member] | Canaccord Genuity LLC [Member] | Common Stock [Member]                  
Class of Stock [Line Items]                  
Remaining availability under financing facility       $ 0     $ 0