Employee benefit plan - Additional Information (Details) - USD ($) $ in Millions |
3 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2020 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Retirement Benefits [Abstract] | ||||
| Defined contribution plan, vest percentage | 10000000.00% | 100.00% | 20000000.00% | 20000000.00% |
| Company contributions to 401 (k) plan | $ 0.3 | |||