Stock option plan - Valuation assumptions (Details) - 2019 Equity Incentive Plan	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock option plan
Expected volatility, minimum	78.90%	73.50%
Expected volatility, maximum	83.80%	74.50%
Risk-free rate, minimum	0.36%	1.53%
Risk-free rate, maximum	1.43%	2.63%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term in years			8 years 2 months 12 days
Minimum
Stock option plan
Expected volatility			71.50%
Risk-free rate			2.90%
Expected term in years	5 years 6 months	6 years 29 days
Maximum
Stock option plan
Expected term in years	6 years 29 days	7 years 7 months 2 days