Income taxes - Changes in valuation allowance (Details) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2018 |
|
| Changes in the valuation allowance | |||
| Balance at the beginning of the year | $ 41,646 | $ 33,810 | $ 34,990 |
| Decrease credited to operations | (3) | (7) | (1,180) |
| Additions charged to operations | 4,816 | 7,843 | |
| Balance at the end of the year | $ 46,459 | 41,646 | $ 33,810 |
| Period of cumulative results for determination of releasing valuation allowance | 3 years | ||
| Uncertain tax positions | $ 0 | $ 0 | |