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Share Capital - Additional Information (Detail)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
May 05, 2016
USD ($)
$ / shares
shares
Apr. 30, 2018
USD ($)
$ / shares
shares
Apr. 30, 2017
USD ($)
$ / shares
shares
May 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
Vote
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
Class of Stock [Line Items]              
Equity investment aggregate purchase price | $         $ 60,000 $ 93,509 $ 30,000
Net proceeds from sale of ordinary shares | $         $ 60,000 $ 93,509 $ 30,000
Common stock, par value | $ / shares         $ 0 $ 0  
Ordinary share holder, voting right         One vote for each ordinary share    
Voting rights, votes per share | Vote         1    
Preferred stock liquidation preference, per share | $ / shares         $ 10    
Singapore [Member]              
Class of Stock [Line Items]              
Common stock, par value | $ / shares            
Common stock, authorized | shares         0    
Series A Preferred Shares [Member] | Singapore [Member]              
Class of Stock [Line Items]              
Preferred stock, par value | $ / shares            
Preferred stock, shares authorized | shares         0    
Follow-On Underwritten Public Offering [Member]              
Class of Stock [Line Items]              
Shares issued under equity agreement | shares     4,166,667        
Purchase price per share | $ / shares     $ 24.00        
Gross proceeds from sale of ordinary shares | $     $ 100,000        
Net proceeds from sale of ordinary shares | $     $ 93,500        
Research, License and Option Agreement [Member] | Pfizer Inc. [Member]              
Class of Stock [Line Items]              
Shares issued under equity agreement | shares 1,875,000     1,875,000      
Purchase price per share | $ / shares $ 16.00     $ 16.00      
Equity investment aggregate purchase price | $ $ 30,000     $ 30,000      
Collaboration and Share Purchase Agreements [Member] | Takeda [Member]              
Class of Stock [Line Items]              
Shares issued under equity agreement | shares   1,096,892          
Equity investment aggregate purchase price | $   $ 60,000          
Net proceeds from sale of ordinary shares | $   $ 60,000          
Purchase price per share | $ / shares   $ 54.70