Organization and Principal Activities - Additional Information (Details) |
1 Months Ended | 12 Months Ended | |||||||
|---|---|---|---|---|---|---|---|---|---|
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Oct. 01, 2018
USD ($)
$ / shares
shares
|
Sep. 14, 2018 |
Jul. 31, 2018 |
Dec. 31, 2018
USD ($)
Segment
shares
|
Dec. 31, 2017
USD ($)
shares
|
Dec. 31, 2016
USD ($)
shares
|
Aug. 31, 2018
USD ($)
|
Aug. 31, 2017
USD ($)
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Dec. 31, 2015
shares
|
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| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Date of incorporation | Apr. 21, 2003 | ||||||||
| Headquartered in state, country name | South San Francisco, California | ||||||||
| Number of operating segments | Segment | 1 | ||||||||
| Net proceeds from issuance of initial public offering | $ 74,436,000 | $ 0 | $ 0 | ||||||
| Gross proceeds from issuance of private placement | $ 10,000,000 | 0 | $ 0 | ||||||
| Reverse split of common stock | Company effected a reverse split of all shares of its common stock at a ratio of 36.3-for-1. Upon the effectiveness of the reverse stock split, (i) all shares of outstanding common stock were adjusted; (ii) the number of shares of common stock for which each outstanding option to purchase common stock is exercisable were adjusted; (iii) the exercise price of each outstanding option to purchase common stock were adjusted; (iv) the conversion ratio for each share of outstanding redeemable convertible preferred stock which is convertible into the Company’s common stock was proportionately reduced; (v) the number of shares of common stock for which each outstanding warrant to purchase common stock is exercisable was proportionally decreased; (vi) the conversion ratio for each outstanding warrant to purchase redeemable convertible preferred stock which is convertible into warrants to purchase the Company’s common stock after the offering was proportionally decreased; and (vii) the exercise price of each outstanding warrant was proportionally increased. All of the outstanding common stock share numbers (including shares of common stock subject to the Company’s options, as converted for the outstanding redeemable convertible preferred stock shares and warrants), share prices, exercise prices and per share amounts contained in the financial statements have been retroactively adjusted in the financial statements to reflect this reverse stock split for all periods presented. The par value per share and the authorized number of shares of common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. | ||||||||
| Accumulated deficit | $ 150,328,000 | $ 115,011,000 | |||||||
| Unrestricted cash, cash equivalents and marketable securities | $ 204,500,000 | ||||||||
| Substantial doubt about going concern, within one year | false | ||||||||
| Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | August 2017 Loan | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Borrowed amount | $ 15,000,000 | $ 15,000,000 | |||||||
| Minimum | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Going concern period from issuance date of unaudited interim condensed financial statements | 1 year | ||||||||
| Common Stock | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Issuance of common stock, shares | shares | 5,667,000 | ||||||||
| Stock split conversion ratio | 36.3 | ||||||||
| Common Stock | Merck | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Gross proceeds from issuance of private placement | $ 10,000,000 | ||||||||
| Common Stock | IPO | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Common stock price, per share | $ / shares | $ 15.00 | ||||||||
| Issuance of common stock, shares | shares | 5,667,000 | ||||||||
| Gross proceeds from issuance of initial public offering | $ 85,000,000 | ||||||||
| Net proceeds from issuance of initial public offering | $ 74,400,000 | ||||||||
| Common Stock | Private Placement | Merck | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Common stock price, per share | $ / shares | $ 15.00 | ||||||||
| Issuance of common stock, shares | shares | 666,666 | ||||||||
| Series E Redeemable Convertible Preferred Stock | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Stock split conversion ratio | 0.8374 | 36.3636 | |||||||
| Stock split description | Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-1.1940912491 split (“Split”) of shares of the Company’s Series E redeemable convertible preferred stock, which was effected on July 26, 2018. | ||||||||
| Redeemable Convertible Preferred Stock | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Redeemable convertible preferred stock, shares outstanding | shares | 0 | 173,750,421 | 173,750,421 | 173,750,421 | |||||
| Redeemable Convertible Preferred Stock | IPO | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Redeemable convertible preferred stock, shares outstanding | shares | 0 | ||||||||
| Redeemable Convertible Preferred Stock | Common Stock | IPO | |||||||||
| Organization Consolidation And Presentation Of Financial Statements [Line Items] | |||||||||
| Convertible preferred stock converted into common stock | shares | 16,028,462 | ||||||||