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Stock-based Compensation - Additional Information (Details) $ / shares in Units, $ in Millions |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended | 24 Months Ended |
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Jul. 02, 2020 shares | Jun. 08, 2020 USD ($) employee | Jan. 01, 2019 USD ($) item | Sep. 30, 2020 USD ($) shares | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) shares | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) employee $ / shares shares | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 USD ($) employee | Dec. 31, 2017 USD ($) employee | Jul. 31, 2015 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense |  |  |  | $ 2.7 | $ 1.5 | $ 7.3 | $ 2.3 | $ 4.3 | $ 2.1 |  |  |
Proceeds from release of shares upon repayment |  |  |  |  |  | $ 1.3 |  |  |  |  |  |
Settlement of subscriptions receivable, number of executive employees | employee |  | 2 |  |  |  |  |  |  |  |  |  |
2020 Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock reserved for issuance (shares) | shares | 5,503,678 |  |  |  |  |  |  |  |  |  |  |
Common stock reserved for issuance, annual increase, percentage of outstanding shares | 5.00% |  |  |  |  |  |  |  |  |  |  |
Maximum shares that may be issued upon exercise of incentive stock options (shares) | shares | 3,650,000 |  |  |  |  |  |  |  |  |  |  |
Common stock available for grant (shares) | shares |  |  |  | 5,262,053 |  | 5,262,053 |  |  |  |  |  |
Shares available for future grant, transferred from previous plan (shares) | shares | 1,753,678 |  |  |  |  |  |  |  |  |  |  |
2020 ESPP |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock reserved for issuance, annual increase, percentage of outstanding shares | 1.00% |  |  |  |  |  |  |  |  |  |  |
Number of shares available for issuance, annual increase (shares) | shares | 1,000,000 |  |  |  |  |  |  |  |  |  |  |
2015 Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock reserved for issuance (shares) | shares |  |  |  |  |  |  |  |  |  |  |  | 7,312,590
Common stock reserved for issuance, annual increase, percentage of outstanding shares |  |  |  |  |  |  |  | 7312590.00% |  |  |  |
Common stock available for grant (shares) | shares |  |  |  | 0 |  | 0 |  | 2,364,832 |  |  |  |
Options to purchase common stock |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense |  |  |  | $ 2.7 | 1.5 | $ 7.3 | 2.3 | $ 4.3 | $ 2.1 |  |  |
Repricing of stock options (shares) | shares |  |  |  |  |  |  |  | 1,708,300 |  |  |  |
Repricing of stock options, number of employees impacted | employee |  |  |  |  |  |  |  | 198 |  |  |  |
Repricing of stock options, exercise price, lower limit (usd per share) | $ / shares |  |  |  |  |  |  |  | $ 26.04 |  |  |  |
Repricing of stock options, exercise price, upper limit (usd per share) | $ / shares |  |  |  |  |  |  |  | 28.53 |  |  |  |
Repricing of stock options, fair value (usd per share) | $ / shares |  |  |  |  |  |  |  | $ 23.69 |  |  |  |
Repricing of stock options, incremental expense |  |  |  | 0.1 |  | 0.3 |  |  |  |  |  |
Repricing of stock options, unrecognized incremental expense |  |  |  | 1.3 |  | $ 1.3 |  |  |  |  |  |
Repricing of stock options, unrecognized incremental expense, period for recognition |  |  |  |  |  | 1 year 1 month 6 days |  |  |  |  |  |
Unrecognized expense |  |  |  | 32.3 |  | $ 32.3 |  | $ 27.3 |  |  |  |
Unrecognized expense, period for recognition |  |  |  |  |  | 1 year 4 months 24 days |  | 1 year 7 months 6 days |  |  |  |
Options to purchase common stock | 2015 Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  | 4 years |  | 4 years |  |  |  |
Expiration period |  |  |  |  |  | 10 years |  | 10 years |  |  |  |
Executive |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Repricing of stock options (shares) | shares |  |  |  |  |  |  |  | 105,487 | 62,500 |  |  |
Repricing of stock options, incremental expense |  |  |  |  |  |  |  | $ 0.2 | $ 0.1 |  |  |
Unrecognized expense |  |  |  |  |  |  |  | $ 0.2 |  |  |  |
Unrecognized expense, period for recognition |  |  |  |  |  |  |  | 1 year 3 months 18 days |  |  |  |
Executive | Options to purchase common stock |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense |  |  |  | 0.1 | $ 0.1 | $ 0.1 | $ 0.1 | $ 0.2 | 0.2 |  |  |
Unrecognized expense |  |  |  | $ 0.1 |  | $ 0.1 |  |  |  |  |  |
Unrecognized expense, period for recognition |  |  |  |  |  | 7 months 6 days |  |  |  |  |  |
Subscriptions receivable, number of executive employees |  |  | 2 |  |  |  |  |  |  | 2 |  |
Subscriptions receivable |  |  |  |  |  |  |  |  |  | $ 1.5 | $ 1.5 |
Subscriptions receivable, weighted average interest rate |  |  |  |  |  |  |  |  |  | 1.90% | 1.90% |
Proceeds from release of shares upon repayment |  | $ 1.3 | $ 1.3 |  |  |  |  |  | $ 1.2 |  |  |
Settlement of subscriptions receivable, number of executive employees | employee |  |  |  |  |  |  |  |  |  |  | 2 |

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