STOCK BASED COMPENSATION - Summary of Valuation Assumptions Used (Details) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants | |||
| Risk-free interest rate | 1.50% | 1.70% | 1.80% |
| Volatility | 76.30% | 77.00% | 77.10% |
| Expected term (in years) | 6 years 7 days | 6 years 2 months 12 days | 6 years 3 months |
| Total stock-based compensation expense | $ 5,787 | $ 2,209 | $ 22 |
| Research and development | |||
| Weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants | |||
| Total stock-based compensation expense | 638 | 223 | 12 |
| General and administrative | |||
| Weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants | |||
| Total stock-based compensation expense | $ 5,149 | $ 1,986 | $ 10 |