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Stockholders' Equity - Additional Information (Detail)
 1 Months Ended 9 Months Ended
Aug. 31, 2016
USD ($)
shares
Jan. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2012
USD ($)
$ / shares
shares
Sep. 30, 2018
USD ($)
$ / shares
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
$ / shares
Class of Stock [Line Items]            
Net proceeds from sale of common stock $ 215,900,000 $ 281,600,000   $ 6,044,000 $ 26,397,000  
Share of common stock issued | shares 6,969,696 10,344,827 19,000,000      
Common stock, par value | $ / shares       $ 0.0001   $ 0.0001
Value of shares purchased by Baker Entities in public offerings       $ 12,000   $ 12,000
Proceed from sale of stock and warrants     $ 80,500,000      
Common stock sale price per share | $ / shares     $ 4.43      
Expire on December 17, 2019            
Class of Stock [Line Items]            
Warrants issued | shares     500,000      
Warrants issuance price per share | $ / shares     $ 4.42      
Warrant exercise price | $ / shares     $ 0.01      
Warrant expiration date       Dec. 17, 2019    
Issuance of warrant value     $ 2,200,000      
Warrants outstanding | shares     500,000      
Description of warrant       Per their terms, the warrants to purchase 500,000 shares of common stock, all of which remained outstanding at September 30, 2018, may not be exercised if the holder’s ownership of the Company’s common stock would exceed 19.99 percent following such exercise.    
Expire on December 17, 2019 | Risk Free Interest Rate            
Class of Stock [Line Items]            
Warrants outstanding, measurement input     1.1      
Expire on December 17, 2019 | Volatility Percentage            
Class of Stock [Line Items]            
Warrants outstanding, measurement input     105.8      
Expire on December 17, 2019 | Warrant Term            
Class of Stock [Line Items]            
Warrant term     7 years      
Expire on December 17, 2019 | Dividend Yield            
Class of Stock [Line Items]            
Warrants outstanding, measurement input     0      
Registration Rights Agreement            
Class of Stock [Line Items]            
Common stock, par value | $ / shares   $ 0.0001        
Expiration period   10 years        
Maximum amount of expense agreed to bear under the agreement   $ 50,000        
Registration Rights Agreement | Baker Brothers Life Sciences L.P.            
Class of Stock [Line Items]            
Value of shares purchased by Baker Entities in public offerings $ 43,000,000 $ 75,000,000        
Option To Purchase Additional Shares            
Class of Stock [Line Items]            
Share of common stock issued | shares 909,090