Annual report pursuant to Section 13 and 15(d)

Convertible Preferred Stock Warrants and Common Stock Warrants - Additional Information (Details)

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Convertible Preferred Stock Warrants and Common Stock Warrants - Additional Information (Details) - USD ($)
 1 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Mar. 28, 2017
Class Of Warrant Or Right [Line Items]          
Loss on remeasurement of convertible preferred stock warrant liability     $ 3,725,000 $ 524,000  
Reclassification of convertible preferred stock warrant liability into additional paid-in capital $ 5,700,000   5,711,000    
Net proceeds from Initial public offering excluding underwriting discounts and commissions 32,400,000   $ 32,426,000    
Fair value of warrants $ 300,000     $ 1,875,000  
The March 28, 2017 Preferred stock warrant | Series B-1 Convertible Preferred Stock          
Class Of Warrant Or Right [Line Items]          
Number of shares called by warrant 61,363       76,704
Class of warrant, exercise price         $ 5.8667
Fair value of warrants         $ 111,000
Warrants Not Settleable in Cash          
Class Of Warrant Or Right [Line Items]          
Conversion of preferred stock into common stock, shares   25,145      
Maximum          
Class Of Warrant Or Right [Line Items]          
Number of shares called by warrant 84,000        
Minimum          
Class Of Warrant Or Right [Line Items]          
Net proceeds from Initial public offering excluding underwriting discounts and commissions $ 25,000,000        
Other Expense, Net          
Class Of Warrant Or Right [Line Items]          
Loss on remeasurement of convertible preferred stock warrant liability   $ 3,700,000