Stock based awards - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Options
Balance Outstanding, Beginning of the period (in shares)	1,839,913
Options granted ( in shares)	1,912,352	1,883,067
Options exercised ( in shares)	(655,710)	(12,586)
Options cancelled (in shares)		(30,568)
Balance Outstanding, End of the period (in shares)	3,096,555	1,839,913
Exercisable ( in shares)	175,300
Vested and expected to vest ( in shares)	3,096,555
Weighted-Average Exercise Price per Share
Weighted-average exercise price, Beginning of the period (in dollars per share)	$ 0.61
Weighted-average exercise price, Granted (in dollars per share)	12.42	$ 0.61
Weighted-average exercise price, Exercised (in dollars per share)	0.69	0.61
Weighted-Average Exercise, cancelled (in dollars per share)		0.61
Weighted-average exercise price, End of the period (in dollars per share)	7.89	$ 0.61
Weighted-average exercise price, Exercisable ( in dollars per share )	3.54
Weighted-average exercise price, Vested and expected to vest at end of the period (in dollars per share)	$ 7.89
Weighted-Average remaining contractual term (years) and Aggregate Intrinsic Value
Weighted-average remaining contractual term outstanding (in years)	9 years 2 months 9 days	9 years 8 months 27 days
Weighted-average remaining contractual term , Options Exercisable(in years)	8 years 10 months 17 days	9 years 8 months 27 days
Weighted-average remaining contractual term vested and expected to vest (in years)	9 years 2 months 9 days
Aggregate Intrinsic Value Balance outstanding	$ 44,323	$ 10,577
Aggregate Intrinsic Value Exercisable	3,270
Aggregate Intrinsic Value Vested and Expected to vest	$ 44,323
Weighted average grant-date fair value per share of stock options granted	$ 8.34