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Stockholders' Equity - Common stock (Details) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 21, 2018
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Class Of Stock [Line Items]              
Sale of common stock, net   $ 64,000 $ 1,873,000        
Common stock issued (in shares)         3,450,000    
Proceeds from sale of common stock, net         $ 0 $ 1,984,000  
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