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Equity - Common Stock - Additional Information (Detail)
$ / shares in Units, $ in Thousands, £ in Millions
 12 Months Ended
Dec. 13, 2019
GBP (£)
shares
Jul. 29, 2019
USD ($)
Jul. 18, 2019
$ / shares
shares
Nov. 29, 2018
USD ($)
$ / shares
shares
Mar. 05, 2018
shares
Feb. 01, 2018
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 13, 2019
$ / shares
Stockholders Equity Note [Line Items]                  
Accrued expenses and other current liabilities             $ 139,826 $ 84,171  
Net proceeds from public offering             440,108 $ 264,840  
American Depositary Shares                  
Stockholders Equity Note [Line Items]                  
Issuance of stock (shares) | shares     22,222,223 11,111,112   19,178,082      
Sale of stock price per share | $ / shares     $ 18.00 $ 17.575   $ 3.41      
Commissions, price per share | $ / shares     $ 17.235            
Over allotment option, exercisable period     30 days     30 days      
Net proceeds from public offering       $ 194,800   $ 70,000      
American Depositary Shares | Over-Allotment Option                  
Stockholders Equity Note [Line Items]                  
Issuance of stock (shares) | shares           2,876,712      
Issuance of stock (shares), exercised | shares     3,333,333   1,438,356        
Gross proceeds from public offering including exercised option   $ 460,000              
Net proceeds from public offering including exercised option   $ 440,100              
Food and Drug Administration                  
Stockholders Equity Note [Line Items]                  
Milestone payment achieved | £ £ 5                
Issuance of common stock for milestone payment (in shares) | shares 257,713                
Shares issued, price per share | $ / shares                 $ 24.12
Accrued expenses and other current liabilities             $ 2,200