Goodwill and intangible assets - Intangible assets rollforward (Details) - USD ($) $ in Thousands |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Changes in intangible assets | ||
| Balance at beginning of year | $ 10,263 | $ 13,783 |
| Additions | 12,640 | |
| Amortization expense | (3,751) | (3,520) |
| Balance at end of year | $ 19,152 | $ 10,263 |