Stock-based compensation - Activity under stock option plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Number of Shares
Outstanding at beginning of period (in shares)	8,342,429
Granted (in shares)	1,917,634
Exercised (in shares)	(26,951)
Cancelled (in shares)	(204,371)
Outstanding at end of period (in shares)	10,028,741
Vested and expected to vest (in shares)	10,028,741
Options exercisable, at end of period (in shares)	4,307,274
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 11.25
Granted (in dollars per share)	6.05
Exercised (in dollars per share)	3.58
Cancelled (in dollars per share)	12.74
Outstanding at end of period (in dollars per share)	10.25
Vested and expected to vest (in dollars per share)	10.25
Options exercisable, at end of period (in dollars per share)	8.14
Additional Disclosures
Weighted-average grant date fair value of options granted (in dollars per share)	$ 4.39	$ 14.51
Cash received from the exercise of stock options	$ 96	$ 764