Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands |
Dec. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|---|---|
| Deferred tax assets: | ||||
| Net operating loss carryforwards | $ 97,791 | $ 61,160 | ||
| Research and development tax credit carryforwards | 23,524 | 13,525 | ||
| Accrued expenses | 2,241 | 1,142 | ||
| Capitalized intellectual property costs | 1,123 | 1,073 | ||
| Capitalized research and development expense | 97 | 109 | ||
| Operating lease liabilities | 11,394 | 12,952 | ||
| Stock-based compensation expense | 15,847 | 9,156 | ||
| Total deferred tax assets | 152,017 | 99,117 | ||
| Deferred tax liabilities: | ||||
| Operating lease assets | (11,050) | (12,720) | ||
| Depreciation and other | (1,090) | (513) | ||
| Total deferred tax liabilities | (12,140) | (13,233) | ||
| Valuation allowance | $ (139,877) | $ (85,884) | $ (33,666) | $ (12,575) |