Stock-based compensation - Stock option valuation (Details) |
6 Months Ended | |
|---|---|---|
Jun. 30, 2020 |
Jun. 30, 2019 |
|
| Assumptions used in the Black-Scholes option-pricing model | ||
| Risk-free interest rate | 0.44% | 2.45% |
| Expected term (in years) | 6 years | 6 years 1 month 6 days |
| Expected volatility | 79.00% | 73.50% |