Stock option plan - Valuation assumptions (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock option plan
Expected volatility, minimum	73.50%
Expected volatility		71.50%
Expected volatility, maximum	74.50%
Risk‑free rate, minimum	1.53%
Risk‑free rate		2.90%
Risk‑free rate, maximum	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			71.90%
Risk‑free rate			2.30%
Expected term in years	6 years 29 days		8 years 7 months 6 days
Maximum
Stock option plan
Expected volatility			75.40%
Risk‑free rate			2.50%
Expected term in years	7 years 7 months 2 days		9 years 8 months 12 days