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Stockholders' Equity - Pre-Funded Series D Warrant Exercises - Additional Information (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 21, 2018
Stockholders Equity Note [Line Items]        
Warrants exercised 11,285 6,536 1  
Pre-Funded Series D Warrants [Member]        
Stockholders Equity Note [Line Items]        
Warrants exercised   5,379    
Pre-Funded Series D Warrants [Member] | Maximum [Member]        
Stockholders Equity Note [Line Items]        
Maximum offering and selling of stock and securities that entity make to raise capital       $ 100.0