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Common Stock - Additional Information (Detail) | 1 Months Ended | 12 Months Ended | 20 Months Ended | 133 Months Ended
--- | --- | --- | --- | ---
Apr. 30, 2018 USD ($) $ / shares shares | Jan. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) Vote shares | Dec. 31, 2018 shares | Dec. 31, 2018 USD ($) shares
Class Of Stock [Line Items] |  |  |  |  |
Number of votes entitled to each share of common stock | Vote |  |  | 1 |  |
Cash dividends | $ |  |  |  |  | $ 0
Proceeds from sale of common stock, net of issuance costs | $ | $ 28,900,000 | $ 53,800,000 | $ 82,666,000 |  |
Common Stock |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |
Unvested shares of common stock |  |  | 118,679 | 118,679 | 118,679
Number of shares forfeited |  |  |  | 86,581 |
Repurchase option exercisable period |  |  | 4 years |  |
Repurchase options share exercised |  |  |  | 41,819 |
Stock sold and issued to investors | 3,280,000 | 5,899,500 |  |  |
Stock sold and issued to investors, per share | $ / shares | $ 9.15 | $ 9.75 |  |  |

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