Stockholders' Equity - Stock-Based Compensation (Details) - USD ($) $ in Thousands |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2019 |
Jun. 30, 2018 |
Jun. 30, 2019 |
Jun. 30, 2018 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Employee Benefits and Share-based Compensation | $ 813 | $ 72 | $ 1,611 | $ 1,209 |
| Research and Development Expense [Member] | ||||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Employee Benefits and Share-based Compensation | 184 | (392) | 380 | 288 |
| General and administrative | ||||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Employee Benefits and Share-based Compensation | $ 629 | $ 464 | $ 1,231 | $ 921 |