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Stockholders' Equity - Additional Information (Details) |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
May 21, 2020 USD ($) $ / shares shares | Sep. 06, 2018 $ / shares shares | May 21, 2018 $ / shares shares | Jan. 04, 2018 USD ($) | Dec. 03, 2015 USD ($) $ / shares shares | Jan. 31, 2018 shares | Nov. 30, 2017 shares | Jun. 30, 2020 USD ($) shares | Mar. 31, 2019 USD ($) shares | Sep. 30, 2020 USD ($) vote $ / shares shares | Sep. 30, 2019 USD ($) | Dec. 31, 2019 $ / shares shares
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock number of votes per share | vote |  |  |  |  |  |  |  |  |  | 1 |  |
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares) | 9,433,963 |  |  |  |  |  |  |  |  |  |  |
Offering price (in dollars per share) | $ / shares | $ 13.25 |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of common stock | $ | $ 117,100,000 |  |  |  |  |  |  |  |  | $ 117,110,000 | $ 291,311,000 |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 0.0001 |  | $ 0.0001
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs | $ |  |  |  |  |  |  |  | $ 117,094,000 | $ 291,344,000 |  |  |
Shares of common stock, repurchase (in shares) |  |  |  |  |  |  |  |  |  | 3,434,613 |  | 4,648,526
Series A Convertible Preferred Stock | Stock Restriction Agreements |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Shares subject to forfeiture (in shares) |  |  |  |  |  |  | 1,305,427 |  |  |  |  |
Founder |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares) |  | 1,795,023 | 251,547 |  |  |  |  |  |  |  |  |
Percentage of fully diluted share capital |  |  |  | 15.00% |  |  |  |  |  |  |  |
Increased in equity capital amount | $ |  |  |  | $ 300,000,000.0 |  |  |  |  |  |  |  |
Common stock issued price per share | $ / shares |  | $ 9.63 | $ 2.61 |  |  |  |  |  |  |  |  |
Additional shares of restricted stock subject to vesting restrictions (in shares) |  | 1,795,023 | 251,547 |  |  |  |  |  |  |  |  |
Common stock |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Conversion of convertible preferred stock into common stock (in shares) |  |  |  |  |  |  |  |  | 30,493,460 |  |  |
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares) |  |  |  |  |  |  |  | 9,433,963 | 19,837,500 |  |  |
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs | $ |  |  |  |  |  |  |  | $ 1,000 | $ 2,000 |  |  |
Common stock | Stock Restriction Agreements |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unrestricted shares of common stock were subject to service vesting conditions (in shares) |  |  |  |  |  | 4,580,444 |  |  |  |  |  |
Founder Shares |  |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares) |  |  |  |  | 9,160,888 |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  | $ 0.0001 |  |  |  |  |  |  |
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs | $ |  |  |  |  | $ 4,100 |  |  |  |  |  |  |
Common Stock vesting percentage |  |  |  | 50.00% |  |  |  |  |  |  |  |
Common stock vesting period |  |  |  | 5 years |  |  |  |  |  |  |  |

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