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Common Stock - Additional Information (Detail)
 1 Months Ended 12 Months Ended 20 Months Ended 133 Months Ended
Apr. 30, 2018
USD ($)
$ / shares
shares
Jan. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
Vote
shares
Dec. 31, 2018
shares
Dec. 31, 2018
USD ($)
shares
Class Of Stock [Line Items]          
Number of votes entitled to each share of common stock | Vote     1    
Cash dividends | $         $ 0
Proceeds from sale of common stock, net of issuance costs | $ $ 28,900,000 $ 53,800,000 $ 82,666,000    
Common Stock          
Class Of Stock [Line Items]          
Unvested shares of common stock     118,679 118,679 118,679
Number of shares forfeited       86,581  
Repurchase option exercisable period     4 years    
Repurchase options share exercised       41,819  
Stock sold and issued to investors 3,280,000 5,899,500      
Stock sold and issued to investors, per share | $ / shares $ 9.15 $ 9.75