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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, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Options outstanding (in shares) |  | 7,274,540 |  | 7,274,540 |  | 6,706,994
Weighted-average exercise price of stock options outstanding (in dollars per share) |  | $ 21.07 |  | $ 21.07 |  | $ 22.33
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | $ 5,095 | $ 4,884 | $ 9,587 | $ 8,683 |
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) |  |  |  | 18,304 |  |
Proceeds from stock plan |  |  |  | $ 300 |  |
2003 Stock Incentive Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Options outstanding (in shares) |  | 281,024 |  | 281,024 |  |
Weighted-average exercise price of stock options outstanding (in dollars per share) |  | $ 2.79 |  | $ 2.79 |  |
Stock Incentive Plan 2013 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized (in shares) |  | 11,896,613 |  | 11,896,613 |  |
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) |  | 6,993,516 |  | 6,993,516 |  |
Weighted-average exercise price of stock options outstanding (in dollars per share) |  | $ 21.81 |  | $ 21.81 |  |
Research and Development |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | $ 2,589 | 2,453 | $ 5,023 | 4,267 |
General and Administrative |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense |  | $ 2,506 | $ 2,431 | $ 4,564 | $ 4,416 |

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