Share-based compensation - Activity (Details) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 31, 2021 shares	Aug. 31, 2020 USD ($) employee shares	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) $ / shares	Dec. 31, 2020 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Share-based Payment Arrangement, Expense | $		$ 0	$ 1,155	$ 117	$ 2,229	$ 302
Fair value of option price per share | $ / shares				$ 8.69		$ 8.69
Two Officers
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options, Granted		92,169
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Number Of Employees Granted Stock Options | employee		2
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Share-based Payment Arrangement, Expense | $			$ 200	$ 0	$ 200	$ 0
Awards granted	13,500
Unvested restricted stock			0		0
The Plans
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options, Beginning balance					1,472,840
Options, Granted					722,331
Options, Forfeited					(21,127)
Options, Exercised					(237,498)
Options, Ending balance			1,936,546		1,936,546		1,472,840
Options, Exercisable			551,262		551,262
Options, Vested or expected to vest			1,936,546		1,936,546
Weighted Average Exercise price
Beginning price | $ / shares					$ 3.14
Granted | $ / shares					23.61
Forfeited | $ / shares					20.21
Exercised | $ / shares					0.31
Ending price | $ / shares			$ 10.94		10.94		$ 3.14
Exercisable | $ / shares			1.81		1.81
Vested or expected to vest | $ / shares			$ 10.94		$ 10.94
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contractual Term					8 years 8 months 4 days		8 years 6 months 3 days
Weighted Average Remaining Contractual Term, Exercisable					7 years 6 months 21 days
Weighted Average Remaining Contractual Term, Vested or expected to vest					8 years 8 months 4 days
Weighted average grant date fair value | $ / shares					$ 17.77	$ 0.87
Intrinsic value, exercised | $					$ 3,900
Intrinsic value, outstanding | $			$ 27,700		$ 27,700