Stock and employee benefit plans - Weighted-average assumptions used (Details) |
12 Months Ended | |
|---|---|---|
Dec. 31, 2019 |
Dec. 31, 2018 |
|
| Fair value of each employee stock award on the grant date and the assumptions regarding the fair value of the underlying common stock | ||
| Expected volatility | 79.70% | 78.60% |
| Risk-free interest rate | 2.30% | 2.80% |
| Expected term (in years) | 6 years | 6 years 4 days |
| Expected dividend yield | 0.00% | 0.00% |