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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
May 31, 2017 | Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Options outstanding (in shares) |  | 8,610,898 |  | 8,610,898 |  | 7,258,353
Weighted-average exercise price of stock options outstanding (in dollars per share) |  | $ 21.48 |  | $ 21.48 |  | $ 21.48
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | $ 6,070 | $ 5,095 | $ 11,356 | $ 9,587 |
2016 Employee Stock Purchase Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized (in shares) | 800,000 |  |  |  |  |
Payroll deduction discount | 10.00% |  |  |  |  |
Purchase price of common stock percent of the fair market value | 85.00% |  |  |  |  |
Common stock purchased (in shares) |  |  |  | 12,305 |  |
2003 Stock Incentive Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Options outstanding (in shares) |  | 203,138 |  | 203,138 |  |
Weighted-average exercise price of stock options outstanding (in dollars per share) |  | $ 2.98 |  | $ 2.98 |  |
Stock Incentive Plan 2013 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized (in shares) |  | 13,856,781 |  | 13,856,781 |  |
Potential annual increase in shares reserved for future issuance (in shares) |  | 1,960,168 |  | 1,960,168 |  |
Potential annual increase in shares reserved for future issuance as percentage of outstanding share |  |  |  | 4.00% |  |
Options outstanding (in shares) |  | 8,407,760 |  | 8,407,760 |  |
Weighted-average exercise price of stock options outstanding (in dollars per share) |  | $ 21.92 |  | $ 21.92 |  |
Research and Development |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | $ 3,058 | 2,589 | $ 5,785 | 5,023 |
General and Administrative |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | $ 3,012 | $ 2,506 | $ 5,571 | $ 4,564 |

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