Stock-based compensation - Restricted Stock (Details) - Restricted stock units	9 Months Ended
	Sep. 30, 2021 $ / shares shares
Shares
Unvested at the beginning of the period (in shares) | shares	2,649,317
Granted (in shares) | shares	795,962
Vested (in shares) | shares	(1,426,160)
Forfeited (in shares) | shares	(269,140)
Unvested at the end of the period (in shares) | shares	1,749,979
Weighted-average grant date fair value
Unvested at the beginning of the period (in dollars per share) | $ / shares	$ 1.73
Granted (in dollars per share) | $ / shares	2.76
Vested (in dollars per share) | $ / shares	1.40
Forfeited/cancelled (in dollars per share) | $ / shares	2.11
Unvested at the end of the period (in dollars per share) | $ / shares	$ 2.41