Summary of significant accounting policies - Stock-based compensation (Details) - shares shares in Thousands |
12 Months Ended | 60 Months Ended |
|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
|
| Stock options | ||
| Share-based Compensation Arrangement by Share-based Payment Award | ||
| Number of awards made | 0 | 0 |