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Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($) |  |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 06, 2018 | May 21, 2018 | Jan. 04, 2018 | Dec. 03, 2015 | Jan. 31, 2018 | Nov. 30, 2017 | Mar. 31, 2019 | Mar. 31, 2018 | Sep. 30, 2019 | Dec. 31, 2018
Stockholders Equity Deficit [Line Items] |  |  |  |  |  |  |  |  |  |
Common stock, par value |  |  |  |  |  |  |  |  | $ 0.0001 | $ 0.0001
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs |  |  |  |  |  |  | $ 291,344,000 |  |  |
Common stock, voting right |  |  |  |  |  |  |  |  | one |
Shares of common stock, repurchase |  |  |  |  |  |  |  |  | 5,053,169 | 7,482,032
Series A Convertible Preferred Stock | Stock Restriction Agreements |  |  |  |  |  |  |  |  |  |
Stockholders Equity Deficit [Line Items] |  |  |  |  |  |  |  |  |  |
Shares subject to forfeiture |  |  |  |  |  | 1,305,427 |  |  |  |
Founder |  |  |  |  |  |  |  |  |  |
Stockholders Equity Deficit [Line Items] |  |  |  |  |  |  |  |  |  |
Common stock issued shares | 1,795,023 | 251,547 |  |  |  |  |  |  |  |
Percentage of fully diluted share capital |  |  | 15.00% |  |  |  |  |  |  |
Increased in equity capital amount |  |  | $ 300,000,000 |  |  |  |  |  |  |
Common stock issued price per share | $ 9.63 | $ 2.61 |  |  |  |  |  |  |  |
Additional shares of restricted stock subject to vesting restrictions | 1,795,023 | 251,547 |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |
Stockholders Equity Deficit [Line Items] |  |  |  |  |  |  |  |  |  |
Conversion of convertible preferred stock into common stock, shares |  |  |  |  |  |  | 30,493,460 |  | 30,493,460 |
Common stock issued shares |  |  |  |  |  |  | 19,837,500 |  |  |
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs |  |  |  |  |  |  | $ 2,000 |  |  |
Unrestricted shares of common stock were subject to service vesting conditions |  |  |  |  |  |  |  | 4,580,444 |  |
Common Stock | Stock Restriction Agreements |  |  |  |  |  |  |  |  |  |
Stockholders Equity Deficit [Line Items] |  |  |  |  |  |  |  |  |  |
Unrestricted shares of common stock were subject to service vesting conditions |  |  |  |  | 4,580,444 |  |  |  |  |
Founder Shares |  |  |  |  |  |  |  |  |  |
Stockholders Equity Deficit [Line Items] |  |  |  |  |  |  |  |  |  |
Common stock issued shares |  |  |  | 9,160,888 |  |  |  |  |  |
Common stock, par value |  |  |  | $ 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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