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Related Parties - Additional Information (Detail) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Third-Party Investor [Member]        
Related Party Transaction [Line Items]        
Related party transaction amount     $ 100  
Shin Nippon Biomedical Laboratories Ltd. (SNBL) [Member]        
Related Party Transaction [Line Items]        
Payment for contract research services $ 1,300   $ 500 $ 400
Scientific Advisor [Member] | Consulting Agreement [Member]        
Related Party Transaction [Line Items]        
Consulting agreement termination notice period   14 days    
Consulting service expenses   $ 13