Stock based awards - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended
	Dec. 31, 2018	Sep. 30, 2019
Number of Options
Balance Outstanding, Beginning of the period (in shares)		1,839,913
Options granted ( in shares)		1,228,352
Options exercised ( in shares)		(651,031)
Balance Outstanding, End of the period (in shares)	1,839,913	2,417,234
Exercisable ( in shares)		72,910
Vested and expected to vest ( in shares)		2,417,234
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)		7.61
Weighted-average exercise price, Exercised (in dollars per share)		0.65
Weighted-average exercise price, End of the period (in dollars per share)	$ 0.61	4.16
Weighted-average exercise price, Exercisable ( in dollars per share )		3.43
Weighted-average exercise price, Vested and expected to vest at end of the period (in dollars per share)		$ 4.16
Weighted-Average remaining contractual term (years) and Aggregate Intrinsic Value
Weighted-average remaining contractual term outstanding (in years)	9 years 8 months 27 days	9 years 2 months 23 days
Weighted-average remaining contractual term , Options Exercisable(in years)		9 years 1 month 10 days
Weighted-average remaining contractual term vested and expected to vest (in years)		9 years 2 months 23 days
Aggregate Intrinsic Value Balance outstanding	$ 10,577	$ 44,931
Aggregate Intrinsic Value Exercisable		1,409
Aggregate Intrinsic Value Vested and Expected to vest		$ 44,931
Weighted average grant-date fair value per share of stock options granted		$ 4.85