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Equity Line Financing, Controlled Equity Offering and Public Offering of Common Stock - Schedule of Total Sales under 2015, 2016 and 2017 Sales Agreement (Detail) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Feb. 09, 2016
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Stifel, Nicolaus & Company, Incorporated [Member] | 2017 Sales Agreement [Member]        
Sale And Issue Of Shares Under Agreement [Line Items]        
Total shares of common stock sold   1,525,100    
Average price per share   $ 1.27    
Gross proceeds   $ 1,944    
Commissions earned by Stifel or Cantor   58    
Other issuance costs   $ 47    
Cantor Fitzgerald & Co. [Member]        
Sale And Issue Of Shares Under Agreement [Line Items]        
Gross proceeds $ 25,000      
Cantor Fitzgerald & Co. [Member] | 2016 Sales Agreement [Member]        
Sale And Issue Of Shares Under Agreement [Line Items]        
Total shares of common stock sold     8,763,400  
Average price per share     $ 4.70  
Gross proceeds     $ 41,156  
Commissions earned by Stifel or Cantor     1,235  
Other issuance costs     $ 185  
Cantor Fitzgerald & Co. [Member] | 2015 Sales Agreement [Member]        
Sale And Issue Of Shares Under Agreement [Line Items]        
Total shares of common stock sold     5,710,200 2,014,300
Average price per share     $ 3.23 $ 3.25
Gross proceeds     $ 18,454 $ 6,546
Commissions earned by Stifel or Cantor     $ 553 197
Other issuance costs       $ 259