STOCK BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($) $ in Thousands |
3 Months Ended | |
|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2018 |
|
| Stock-based compensation | ||
| Total stock-based compensation expense | $ 325 | $ 26 |
| Research and development | ||
| Stock-based compensation | ||
| Total stock-based compensation expense | 147 | 13 |
| General and administrative | ||
| Stock-based compensation | ||
| Total stock-based compensation expense | $ 178 | $ 13 |