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 Thousands
3 Months Ended 9 Months Ended 30 Months Ended
Oct. 27, 2017
Oct. 21, 2016
Oct. 18, 2016
Apr. 22, 2016
Apr. 19, 2016
Aug. 28, 2015
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Oct. 26, 2017
Aug. 31, 2017
Mar. 31, 2017
Common stock, par value (in dollars per share) $ 0.001           $ 0.001 $ 0.001   $ 0.001     $ 0.001
Reduction of warrants to purchase common stock (as a percent) 19.00%                        
Number of common stock shares available to accredited investors     3,600,000                    
Share price of common stock sold (in dollars per share)     $ 1.20                    
Loss on adjustment of exercise price             $ 84 $ 741          
Net proceeds from offering of common stock and warrants   $ 6,800   $ 13,100       8,293 $ 19,886        
Number of common stock shares available in public offering         2,700,000                
Beneficial ownership of common stock resulting from purchase of stock in public offering (as a percent)         4.99%                
Number of warrants or shares of common stock to be sold to purchase common stock         2                
Aggregate offering price for at-the-market offering program               $ 9,600          
Common stock sold (in shares)             3,400,000 9,200,000   16,500,000      
Net proceeds from sale of the common stock, after deducting fees and other offering expenses (in dollars)             $ 2,600 $ 6,500   $ 19,300      
Common stock commissions paid             100 200   $ 600      
Maximum                          
Aggregate offering price for at-the-market offering program           $ 30,000              
Series A Warrants                          
Number of shares of common stock that could be purchased with warrants         4,100,000           1,928,750    
Exercise price (in dollars per share)     $ 1.34   $ 3.50           $ 0.90 $ 0.60  
Loss on adjustment of exercise price             $ 100 $ 700          
Number of warrants or shares of common stock to be sold to purchase common stock         1                
Number of shares of common stock purchased with each warrant         1                
Length of time from issue date until warrants are exercisable         6 months                
Length of time from exercise date until warrants expire         5 years                
Outstanding warrants (in shares)             8,478,750 8,478,750   8,478,750      
Net proceeds from sale of the common stock, after deducting fees and other offering expenses (in dollars) $ 1,700                        
Series A Warrants | Maximum                          
Number of shares of common stock that could be purchased with warrants     6,300,000                    
Series B Warrants                          
Number of shares of common stock that could be purchased with warrants     2,700,000                    
Exercise price (in dollars per share)     $ 1.20   $ 3.48                
Exercise price pre-funded at closing (in dollars per share)     1.19                    
Exercise price, payable upon exercise (in dollars per share)     $ 0.01                    
Number of warrants or shares of common stock to be sold to purchase common stock         2                
Length of time from exercise date until warrants expire         9 months                
Outstanding warrants (in shares)             0 0   0      
Series B Warrants | Maximum                          
Number of shares of common stock that could be purchased with warrants         5,500,000