Stock Plans - Stock-based compensation expense recorded for awards (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, Compensation Cost | ||||
| Total stock-based compensation expense | $ 350 | $ 402 | $ 486 | $ 834 |
| Research and development | ||||
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost | ||||
| Total stock-based compensation expense | 14 | 156 | 91 | 318 |
| General and administrative | ||||
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost | ||||
| Total stock-based compensation expense | $ 336 | $ 246 | $ 395 | $ 516 |