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Merger Accounting - Additional Information (Detail)
$ in Thousands
3 Months Ended 9 Months Ended
Mar. 13, 2019
USD ($)
shares
Jun. 30, 2019
USD ($)
Sep. 30, 2019
USD ($)
Program
Dec. 31, 2018
USD ($)
Business Acquisition [Line Items]        
Business acquisition, equity interest 64.00%      
Goodwill $ 27,100   $ 27,109 $ 0
Goodwill, Purchase Accounting Adjustments   $ 298    
Business combination, acquisition related costs     $ 1,000  
Number of development programs acquired | Program     3  
Weighted average discount rate     20.00%  
Pro Forma        
Business Acquisition [Line Items]        
Business combination, acquisition related costs     $ 2,700  
Arsanis Security Holders        
Business Acquisition [Line Items]        
Business acquisition, equity interest 31.30%      
XFour Security Holders        
Business Acquisition [Line Items]        
Business acquisition, equity interest 68.70%      
Arsanis        
Business Acquisition [Line Items]        
Business acquisition share exchange ratio 0.5702      
Business combination options to purchase of common stock (in shares) | shares 271,230      
Goodwill $ 27,109