Stock-based compensation - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Shares
Outstanding at the beginning of the period (in shares)	12,690,745
Granted (in shares)	1,652,863
Exercised (in shares)	(565,891)
Forfeited/cancelled (in shares)	(830,708)
Outstanding at the end of the period (in shares)	12,947,009		12,690,745
Vested at the end of the period (in shares)	8,454,091
Weighted-average exercise price per share
Outstanding at the beginning of the period (in dollars per share)	$ 3.90
Granted (in dollars per share)	2.78
Exercised (in dollars per share)	1.59
Forfeited/cancelled (in dollars per share)	3.71
Outstanding at the end of the period (in dollars per share)	3.87		$ 3.90
Vested at the end of the period (in dollars per share)	$ 4.54
Weighted-average remaining contractual term
Outstanding at the end of the period	6 years 1 month 6 days		6 years 6 months
Vested at the end of the period	4 years 9 months 18 days
Aggregate intrinsic value
Outstanding at the beginning of the period (in dollars)	$ 3,390
Outstanding at the end of the period (in dollars)	10,140		$ 3,390
Vested at the end of the period (in dollars)	$ 6,720
Assumptions used to estimate fair value of each stock-based award on the grant date
Risk-free interest rate (as a percent)	0.76%	0.75%
Volatility (as a percent)	95.00%	94.00%
Dividend yield (as a percent)	0.00%	0.00%
Expected term	5 years 10 months 24 days	5 years 9 months 18 days