Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|---|
| Deferred tax assets: | |||
| Net operating loss carryforwards | $ 25,508 | $ 10,422 | |
| Research and development tax credit carryforwards | 3,824 | 1,241 | |
| Accrued expenses | 1,139 | 413 | |
| Capitalized intellectual property costs | 764 | 367 | |
| Capitalized research and development expense | 120 | 131 | |
| Stock-based compensation expense | 2,652 | 375 | |
| Total deferred tax assets | 34,007 | 12,949 | |
| Deferred tax liabilities: | |||
| Depreciation and other | (341) | (374) | |
| Total deferred tax liabilities | (341) | (374) | |
| Valuation allowance | $ (33,666) | $ (12,575) | $ (6,454) |