Stock incentive plans - Stock based compensation expense (Details) - USD ($) $ in Thousands |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Stock-based compensation expense | $ 3,318 | $ 901 |
| Research and development | ||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Stock-based compensation expense | 866 | 325 |
| General and administrative | ||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Stock-based compensation expense | $ 2,452 | $ 576 |