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Stockholders' Equity - Additional Information (Detail) - USD ($)
 12 Months Ended 22 Months Ended
Feb. 15, 2018
Feb. 13, 2018
Dec. 22, 2017
Dec. 20, 2017
Apr. 12, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Mar. 16, 2016
Stockholders Equity Disclosure [Line Items]                    
Common stock, value of shares issued under sales agreement           $ 12,000 $ 6,000 $ 6,000    
Net proceeds from sale of common stock $ 106,800,000   $ 9,700,000   $ 18,600,000   $ 5,336,000      
Warrants to purchase common stock         2,070,000          
Warrants exercise price         $ 5.50 $ 155.73     $ 145.11  
Preferred stock, shares issued           0 3,680 3,680    
Preferred stock, shares outstanding           0 3,680 3,680    
Net proceeds allocated to common stock, preferred A stock and warrants             $ 18,563,000      
Common stock price, per share         $ 5.28          
Proceeds allocated to warrants, net         $ 5,000,000          
Number of warrants outstanding           10,194 1,751,708 1,751,708 12,039  
Overallotment Option                    
Stockholders Equity Disclosure [Line Items]                    
Warrants to purchase common stock         270,000          
April 2017 Underwritten Public Offering                    
Stockholders Equity Disclosure [Line Items]                    
Number of warrants outstanding           0        
Series A Preferred Stock                    
Stockholders Equity Disclosure [Line Items]                    
Issuance of stock, shares         13,350          
Conversion of each preferred stock to common stock         200          
Number of shares converted           3,680 9,670      
Preferred stock, shares issued           0        
Preferred stock, shares outstanding           0 3,680 3,680    
Net proceeds allocated to preferred stock             $ 10,100,000      
Effective conversion price             $ 3.80 $ 3.80    
Intrinsic value of preferred stock         $ 4,000,000          
Effective percentage of conversion rights         100.00%          
Common Stock                    
Stockholders Equity Disclosure [Line Items]                    
Issuance of stock, shares 3,382,352   1,105,263   1,470,000   439,880      
Conversion of preferred stock to common stock           736,000 1,934,000      
Common stock price, per share $ 34.00   $ 9.50              
Firm commitment underwritten public offering shares   2,941,176   1,105,263            
Common Stock | Overallotment Option                    
Stockholders Equity Disclosure [Line Items]                    
Issuance of stock, shares 441,176       540,000          
Capital on Demand Sales Agreement with Jones Trading Institutional Services LLC                    
Stockholders Equity Disclosure [Line Items]                    
Issuance of stock, shares               479,681    
Common stock weighted average selling price               $ 13.55    
Net proceeds from sale of common stock             $ 5,500,000 $ 6,300,000    
Commission against additional paid-in capital               $ 200,000    
Common stock available for sale             0 0    
Capital on Demand Sales Agreement with Jones Trading Institutional Services LLC | Maximum                    
Stockholders Equity Disclosure [Line Items]                    
Common stock, value of shares issued under sales agreement                   $ 6,500,000