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Stockholders' Equity - Common Stock - Additional Information (Detail) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended 20 Months Ended
Jul. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Mar. 17, 2018
Jul. 26, 2018
Mar. 31, 2018
Class Of Stock [Line Items]                
Common stock reserved for future issuance   29,290,172   29,290,172        
Issuance of common stock   117,769,919   117,769,919       111,032,957
Common stock, shares authorized   200,000,000   200,000,000       200,000,000
At-The-Market Facility [Member]                
Class Of Stock [Line Items]                
Issuance of common stock   5,560,514   5,560,514        
Certificate of Amendment [Member]                
Class Of Stock [Line Items]                
Common stock, shares authorized             200,000,000  
Equity Offering Sales Agreement [Member]                
Class Of Stock [Line Items]                
Equity sales agreement expiration date Mar. 17, 2018              
Issuance of common stock           7,304,286    
Value of shares sold under equity distribution agreement     $ 3,100,000   $ 7,100,000 $ 19,900,000    
Issuance of common stock from stock options exercises, net, Shares     2,255,541   3,793,758      
Equity Offering Sales Agreement [Member] | Maximum [Member]                
Class Of Stock [Line Items]                
Common stock reserved for future issuance 26,600,000              
2018 Sales Agreement [Member]                
Class Of Stock [Line Items]                
Equity sales agreement expiration date       Feb. 22, 2021        
Issuance of common stock   5,560,514   5,560,514        
Value of shares sold under equity distribution agreement       $ 6,900,000        
2018 Sales Agreement [Member] | At-The-Market Facility [Member]                
Class Of Stock [Line Items]                
Value of shares sold under equity distribution agreement   $ 1,900,000   $ 6,900,000        
Issuance of common stock from stock options exercises, net, Shares   1,798,384   5,560,514        
Common stock value reserved for future issuance   $ 42,900,000   $ 42,900,000        
2018 Sales Agreement [Member] | Maximum [Member]                
Class Of Stock [Line Items]                
Common stock value reserved for future issuance   $ 92,900,000   $ 92,900,000