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Share-Based Compensation - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share-based compensation expense   $ 12,143,000 $ 6,847,000 $ 4,024,000  
Research and Development Expenses [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share-based compensation expense   $ 7,670,000 $ 4,936,000 $ 2,268,000  
Executive Officer [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Options to purchase common stock granted 190,856        
Restricted Share Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting period   4 years      
Number of units, granted   170,859   0  
Unrecognized stock-based compensation weighted average recognition period   3 years 1 month 9 days      
Unrecognized stock-based compensation expense   $ 3,500,000      
2014 Plan [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Ordinary shares reserved for issuance         1,763,714
Number of shares authorized (in shares)   6,064,544   5,064,544  
Ordinary shares available for future grant   1,716,110      
Options to purchase common stock granted   522,750      
Unrecognized stock-based compensation weighted average recognition period   2 years 7 months 6 days      
Weighted average grant date fair value per granted option   $ 16.58 $ 30.23    
Aggregate fair value of options vested   $ 11,500,000      
Share-based compensation expense $ 900,000        
2014 Plan [Member] | Non-Employees Stock Option [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Options to purchase common stock granted   0 0    
Unrecognized stock-based compensation expense   $ 1,000,000      
2014 Plan [Member] | Non-Employees Stock Option [Member] | Research and Development Expenses [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share-based compensation expense   2,900 $ 2,700 $ 1,600  
2014 Plan [Member] | Employee Stock Option [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Unrecognized stock-based compensation expense   $ 16,900,000      
2014 Plan [Member] | Minimum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting period   3 years      
Contractual life of options   5 years      
2014 Plan [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting period   4 years      
Contractual life of options   10 years