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Stockholders' Equity - Common stock (Details) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 21, 2018
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2019
Class Of Stock [Line Items]          
Sale of common stock, net     $ 1,873,000    
Proceeds from sale of common stock, net   $ 0 $ 1,919,000    
Common Stock [Member]          
Class Of Stock [Line Items]          
Common stock issued (in shares)     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