Quarterly report pursuant to Section 13 or 15(d)

Description of Business and Basis of Preparation - Additional Information (Detail)

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Description of Business and Basis of Preparation - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 14, 2018
Mar. 08, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Jun. 21, 2016
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Shelf registration maximum equity offering price $ 175,000,000            
Aggregate amount receivable upon exercise of warrants 53,000,000            
Common stock issued, net of expenses (in shares)   17,751,500          
Common stock issued, net of expenses     $ 498,000 $ 3,583,000 $ 19,498,000 $ 8,638,000  
Common Stock              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Common stock issued, net of expenses (in shares)     328,871 1,847,320 18,181,064 3,708,293  
Common stock issued, net of expenses     $ 0 $ 2,000 $ 18,000 $ 4,000  
Series 1 Warrant              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Number of warrants issued to purchase common stock (in shares)     115,550   115,550    
Series 2 Warrant              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Exercise price of warrants (in dollars per share)   $ 2.00          
Sales Agreement | Common Stock              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Shelf registration maximum equity offering price $ 25,000,000            
June 2016 Public Offering              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Number of warrants issued to purchase common stock (in shares) 4,218,750            
Exercise price of warrants (in dollars per share) $ 3.00           $ 3.00
March 2018 Public Offering              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Number of warrants issued to purchase common stock (in shares)   21,301,800          
Common stock issued, net of expenses (in shares)   17,751,500          
Share price (in dollars per share)   $ 1.69          
Common stock issued, net of expenses   $ 27,900,000          
March 2018 Public Offering | Series 1 Warrant              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Number of warrants issued to purchase common stock (in shares) 13,198,075            
Exercise price of warrants (in dollars per share) $ 1.85 $ 1.85          
March 2018 Public Offering | Series 2 Warrant              
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]              
Number of warrants issued to purchase common stock (in shares) 7,988,175            
Exercise price of warrants (in dollars per share) $ 2.00