Stockholders' Equity - Stock-Based Compensation (Details) - USD ($) $ in Thousands |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2019 |
Sep. 30, 2018 |
Sep. 30, 2019 |
Sep. 30, 2018 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Employee Benefits and Share-based Compensation | $ 833 | $ 1,127 | $ 2,444 | $ 2,336 |
| Research and Development Expense [Member] | ||||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Employee Benefits and Share-based Compensation | 179 | 642 | 559 | 930 |
| General and administrative | ||||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Employee Benefits and Share-based Compensation | $ 654 | $ 485 | $ 1,885 | $ 1,406 |