Stock option plans - Valuation assumptions (Details) - 2019 Equity Incentive Plan	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Stock option plan
Expected volatility, minimum	84.80%	84.40%	88.10%
Expected volatility, maximum	91.60%	89.60%	90.20%
Risk-free rate, minimum	3.41%	1.95%	0.96%
Risk-free rate, maximum	4.73%	2.69%	1.04%
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Stock option plan
Expected term in years	5 years 6 months	6 years 29 days	5 years 6 months
Maximum
Stock option plan
Expected term in years	10 years	10 years	6 years 29 days