Stock option plans - Summary (Details) - 2019 Equity Incentive Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Stock option plan
Options outstanding	596,466	480,274	229,308	188,552
Shares available for grant	61,637
Stock based compensation expense	$ 0.4	$ 17.0	$ 7.8
Unrecognized compensation cost	$ 0.6
Recognition period for unrecognized compensation cost	2 years 9 months 18 days