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Stock Plan and Stock-Based Compensation - Additional Information (Details) - USD ($) |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 01, 2020 | Jan. 01, 2019 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Unrecognized employee compensation costs |  |  |  | $ 23,700,000 |  |
Non-vested stock option recognized weighted average period |  |  |  | 2 years 10 months 24 days |  |
Intrinsic value of shares exercised |  |  |  | $ 200,000 | $ 600,000 |
Fair value of shares vested |  |  |  | $ 7,800,000 | $ 3,000,000 |
Total shares of authorized common stock reserved for future issuance |  |  |  | 8,310,754 | 5,259,271 |
Common stock, shares outstanding |  |  |  | 45,925,004 | 44,537,946 |
Share-based compensation, vested stock options |  |  |  | 372,774 | 14,918 |
Description of expected term simplified method |  |  |  | The Company has opted to use the “simplified method” for estimating the expected term of options, whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option (generally 10 years). |  |
Expected term |  |  |  | 10 years |  |
Expected dividend yield |  |  |  | 0.00% |  |
Non-Employee Stock-Based Compensation |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares not vested and subject to repurchase |  |  |  | 308,596 | 31,388 |
Stock based compensation expense |  |  |  | $ 900,000 | $ 300,000 |
Restricted Stock Agreements |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Award vesting period |  |  |  |  |  | 4 years
Estimated fair value, price per share |  |  |  |  |  | $ 0.0004
Vesting interval period |  |  |  |  |  | monthly
Chief Executive Officer and President | Restricted Stock Agreements |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares issued |  |  |  |  |  | 2,777,776
Price per share |  |  |  |  |  | $ 0.0004
2018 Equity Incentive Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of shares issued or transferred |  |  | 3,570,000 |  |  |
Total shares of authorized common stock reserved for future issuance |  |  |  | 2,315,099 |  |
2018 Equity Incentive Plan | Maximum |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares of common stock reserved for issuance under evergreen provision |  | 3,570,000 |  |  |  |
Percentage of common stock outstanding |  | 4.00% |  |  |  |
2018 Equity Incentive Plan | Subsequent Event |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares of common stock reserved for issuance under evergreen provision | 1,837,000 |  |  |  |  |
2015 Stock Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares not vested and subject to repurchase |  |  |  | 455,158 | 927,123 |
Cash received from early exercise of unvested options |  |  |  | $ 1,700,000 | $ 2,800,000 |
Award vesting period |  |  |  | 48 months |  |
Total shares of authorized common stock reserved for future issuance |  |  | 709,558 |  |  |
2018 Employee Stock Purchase Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Purchase date of fair market value trading days |  |  | 24 months |  |  |
Total shares of authorized common stock reserved for future issuance |  |  | 714,000 | 933,273 |  |
2018 Employee Stock Purchase Plan | Maximum |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Purchase price of common stock as percentage of market value |  |  | 85.00% |  |  |
Number of shares of common stock that may be purchased by an employee on any purchase date |  |  | 3,000 |  |  |
Number of shares that may be purchased in any calendar year |  |  | $ 25,000 |  |  |
Percentage of common stock shares outstanding |  | 1.00% |  |  |  |
Common stock, shares outstanding |  | 1,071,000 |  |  |  |
2018 Employee Stock Purchase Plan | Subsequent Event |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares of common stock reserved for issuance under evergreen provision | 459,250 |  |  |  |  |

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