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Stockholders' Equity - Common stock (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Oct. 02, 2020
Sep. 30, 2020
Jun. 16, 2020
Sep. 21, 2018
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2019
Dec. 31, 2019
Mar. 16, 2020
Class Of Stock [Line Items]                    
Sale of common stock, net         $ 64,000 $ 1,873,000        
Proceeds from sale of common stock, net               $ 15,964,000    
Common Stock [Member]                    
Class Of Stock [Line Items]                    
Common stock issued (in shares)         5,000,000 139,855,000        
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member]                    
Class Of Stock [Line Items]                    
Sale of common stock, net       $ 5,000,000            
Common stock issued (in shares)                 32,170  
Proceeds from sale of common stock, net                 $ 300,000  
Period exercisable from the date of issuance       24 months            
Remaining availability under financing facility   $ 0                
Common Stock [Member] | Purchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member]                    
Class Of Stock [Line Items]                    
Common stock issued (in shares)             12,802      
Proceeds from sale of common stock, net             $ 300,000      
Common Stock [Member] | 2020 Purchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member]                    
Class Of Stock [Line Items]                    
Sale of common stock, net   $ 25,000,000                
Common stock issued (in shares)   0                
Period exercisable from the date of issuance   36 months                
Closing price per share                   $ 1.05
Common Stock [Member] | 2020 Purchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Subsequent Event [Member]                    
Class Of Stock [Line Items]                    
Common stock issued (in shares) 180,701                  
Fair value of commitment fee $ 500,000                  
Common Stock [Member] | 2020 Purchase Agreement [Member] | Lincoln Park Capital Fund, LLC [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                
Common Stock [Member] | 2020 Purchase Agreement [Member] | Lincoln Park Capital Fund, LLC [Member] | Maximum [Member]                    
Class Of Stock [Line Items]                    
Common stock issued (in shares)     23,800,000              
Percentage issuance of common stock     19.99%              
Common Stock [Member] | 2020 Purchase Agreement [Member] | Lincoln Park Capital Fund, LLC [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