Stock based awards - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Number of Options
Balance Outstanding, Beginning of the period (in shares)	3,096,555
Options granted ( in shares)	103,500
Options exercised ( in shares)	(103,385)
Options cancelled (in shares)	(1,220)
Balance Outstanding, End of the period (in shares)	3,095,450	3,096,555
Exercisable ( in shares)	264,176
Vested and expected to vest ( in shares)	3,095,450
Weighted-Average Exercise Price per Share
Weighted-average exercise price, Beginning of the period (in dollars per share)	$ 7.89
Weighted-average exercise price, Granted (in dollars per share)	20.08
Weighted-average exercise price, Exercised (in dollars per share)	1.09
Weighted-Average Exercise, cancelled (in dollars per share)	0.62
Weighted-average exercise price, End of the period (in dollars per share)	8.53	$ 7.89
Weighted-average exercise price, Exercisable ( in dollars per share )	7.76
Weighted-average exercise price, Vested and expected to vest at end of the period (in dollars per share)	$ 8.53
Weighted-Average remaining contractual term (years) and Aggregate Intrinsic Value
Weighted-average remaining contractual term outstanding (in years)	8 years 11 months 23 days	9 years 2 months 9 days
Weighted-average remaining contractual term , Options Exercisable(in years)	8 years 9 months 29 days
Weighted-average remaining contractual term vested and expected to vest (in years)	8 years 11 months 23 days
Aggregate Intrinsic Value Balance outstanding	$ 39,285	$ 44,323
Aggregate Intrinsic Value Exercisable	3,550
Aggregate Intrinsic Value Vested and Expected to vest	$ 39,285
Weighted average grant-date fair value per share of stock options granted	$ 13.91