Quarterly report pursuant to Section 13 or 15(d)

Offerings of Common Stock and Warrants and At-the-Market Offering Program (Details)

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Offerings of Common Stock and Warrants and At-the-Market Offering Program (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended 42 Months Ended
Apr. 13, 2018
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2018
Jun. 07, 2018
Aug. 28, 2015
Common stock sold (in shares) 3,806,243          
June 2018 Sales Agreement            
Aggregate offering price for at-the-market offering program   $ 17.8 $ 17.8 $ 17.8    
Common stock sold (in shares)   3,900,000 7,600,000      
Net proceeds from sale of the common stock, after deducting fees and other offering expenses (in dollars)   $ 2.9 $ 6.9      
Common stock commissions paid   $ 0.1 $ 0.2      
August 2015 Sales Agreement            
Common stock sold (in shares)     2,800,000 26,000,000    
Net proceeds from sale of the common stock, after deducting fees and other offering expenses (in dollars)     $ 4.0 $ 28.6    
Common stock commissions paid     $ 0.1 $ 0.8    
Maximum | June 2018 Sales Agreement            
Aggregate offering price for at-the-market offering program         $ 25.0  
Maximum | August 2015 Sales Agreement            
Aggregate offering price for at-the-market offering program           $ 30.0
Series A Warrants            
Warrants exercised (in shares) 5,760,000          
Exercise price (in dollars per share) $ 0.60          
Outstanding warrants (in shares)   2,718,750 2,718,750 2,718,750    
Series A Warrants, Exercise Price 2.55            
Exercise price (in dollars per share)   $ 2.55 $ 2.55 $ 2.55    
Issued warrants (in shares)     2,178,750      
Series A Warrants, Exercise Price 1.34            
Exercise price (in dollars per share)   0.57 $ 0.57 0.57    
Initial exercise price (in dollars per share)   $ 1.34 $ 1.34 $ 1.34    
Issued warrants (in shares)     540,000      
Series B Warrants            
Outstanding warrants (in shares)   0 0 0