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Liquidity and Going Concern - Additional Information (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Oct. 23, 2020
Sep. 30, 2020
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Liquidity [Line Items]                      
Net loss     $ (3,724,000) $ (2,800,000) $ (2,720,000) $ (1,728,000) $ (1,839,000) $ (1,087,000) $ (9,244,000) $ (4,654,000)  
Accumulated deficit     (442,755,000)           (442,755,000)   $ (433,511,000)
Net cash used in operating activities                 $ (7,656,000) (5,210,000)  
Substantial doubt about going concern, description                 These factors raise substantial doubt about the Company’s ability to continue as a going concern.    
Proceeds from sale of common stock                 $ 18,665,000 0  
Sale of common stock, net     $ 6,354,000 $ 5,094,000 $ 7,078,000            
Common Stock [Member]                      
Liquidity [Line Items]                      
Common stock issued (in shares)     3,272,500 1,907,000 2,534,879            
Sale of common stock, net     $ 3,000 $ 2,000 $ 2,000            
Underwriter [Member] | Common Stock [Member]                      
Liquidity [Line Items]                      
Proceeds from sale of common stock                 $ 6,300,000   $ 3,200,000
Common stock issued (in shares)                 2,179,193   1,616,331
Remaining common stock issued and sold (in shares)     0           0    
Canaccord Genuity LLC [Member] | Common Stock [Member]                      
Liquidity [Line Items]                      
Proceeds from sale of common stock $ 10,000,000               $ 6,300,000   $ 3,200,000
Remaining availability under financing facility $ 0                    
Common stock issued (in shares)                 2,179,193   1,616,331
Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member]                      
Liquidity [Line Items]                      
Sale of common stock, net                   $ 25,000,000.0  
Period exercisable from the date of issuance                   36 months  
Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member] | 2020 Repurchase Agreement [Member]                      
Liquidity [Line Items]                      
Proceeds from sale of common stock                 $ 12,300,000   $ 700,000
Common stock issued (in shares)                 5,535,186   353,113
Sale of common stock, net   $ 25,000,000.0                  
Period exercisable from the date of issuance   36 months                  
Common stock issued as commitment fee (in shares)                     180,701