Stock options - Activity under stock option plan (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) $ / shares
Number of Shares
Outstanding at beginning of period (in shares) | shares	6,112,948
Granted (in shares) | shares	2,957,970
Exercised (in shares) | shares	(331,141)
Cancelled (in shares) | shares	(524,228)
Outstanding at end of period (in shares) | shares	8,215,549
Options exercisable, at end of period (in shares) | shares	3,731,070
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 7.84
Granted (in dollars per share)	17.73
Exercised (in dollars per share)	4.70
Cancelled (in dollars per share)	11.33
Outstanding at end of period (in dollars per share)	11.31
Options exercisable, at end of period (in dollars per share)	6.76
Additional Disclosures
Weighted-average grant date fair value of options granted (in dollars per share)	$ 12.37	$ 6.17
Cash received from the exercise of stock options | $	$ 1,545	$ 1,732