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The Company and Basis of Presentation - Additional Information (Details)
1 Months Ended 9 Months Ended
Oct. 31, 2018
USD ($)
shares
Apr. 30, 2018
USD ($)
$ / shares
shares
May 31, 2017
USD ($)
Sep. 30, 2018
USD ($)
Segment
Dec. 31, 2017
USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Number of operating segments | Segment       1  
Working capital       $ 62,900,000  
Accumulated deficit       (101,963,000) $ (72,513,000)
Cash, cash equivalents and marketable securities       $ 64,700,000  
At-the-Market (ATM) Sales Agreement | JonesTrading Institutional Services LLC | Subsequent Event          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Number of shares issued | shares 1,845,820        
Net proceeds from sale of common stock $ 16,400,000        
At-the-Market (ATM) Sales Agreement | JonesTrading Institutional Services LLC | Maximum          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Aggregate offering price of common stock     $ 20,000,000    
Common Stock          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Number of shares issued | shares   5,046,510      
Public offering price | $ / shares   $ 8.00      
Proceeds from issuance of common stock upon initial public offering, net   $ 37,700,000      
Underwriting discounts and commissions   2,400,000      
Offering costs   $ 300,000      
Common Stock | Underwriters' Over-allotment Option          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Number of shares issued | shares   546,510