Stock option plan - Stock option activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of options
Outstanding, beginning of period	1,844,188	1,897,206	1,237,292
Granted	2,018,796	24,067	663,224
Exercised	(322,267)	(2,306)
Cancelled/Forfeited	(40,783)	(74,779)	(3,310)
Outstanding, end of period	3,499,934	1,844,188	1,897,206	1,237,292
Exercisable	1,203,298
Vested or expected to vest	3,499,934
Weighted‑average exercise price per share
Outstanding, beginning of period	$ 0.82	$ 1.46	$ 1.85
Granted	8.82	1.01	0.73
Exercised	0.23	0.92
Cancelled/Forfeited	0.99	17.31	0.92
Outstanding, end of period	5.49	$ 0.82	$ 1.46	$ 1.85
Exercisable	0.93
Vested or expected to vest	$ 5.49
Weighted average remaining contractual term (in years)
Outstanding	7 years 6 months	7 years 7 months 6 days	8 years 3 months 18 days	9 years 2 months 12 days
Exercisable	6 years 8 months 12 days
Vested or expected to vest	7 years 6 months
Aggregate Intrinsic Value
Outstanding	$ 141,413,705
Exercisable	54,101,434
Vested or expected to vest	$ 141,413,705
Additional Disclosures
Weighted average grant date fair value of options granted	$ 6.68	$ 0.69	$ 0.75