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Stockholders' Equity - Additional Information (Detail)
 1 Months Ended 3 Months Ended
Nov. 30, 2018
USD ($)
shares
Aug. 31, 2016
USD ($)
shares
Jan. 31, 2016
USD ($)
shares
Dec. 31, 2012
USD ($)
$ / shares
shares
Mar. 31, 2019
USD ($)
shares
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Class of Stock [Line Items]              
Net proceeds from sale of common stock $ 298,500,000 $ 215,900,000 $ 281,600,000   $ 4,176,000 $ 2,880,000  
Share of common stock issued | shares 18,602,941 6,969,696 10,344,827 19,000,000      
Value of shares purchased by Baker Entities in public offerings         $ 14,000   $ 14,000
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       500,000 493,145    
Description of warrant         Per their terms, the warrants to purchase 500,000 shares of common stock, of which 493,145 remained outstanding at March 31, 2019, 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]              
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 2,426,470 909,090