Commitments and Contingencies - Components of lease expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Commitments and Contingencies
Operating lease cost	$ 273	$ 226	$ 773	$ 669
Variable lease cost	222	153	444	456
Total lease cost	$ 495	$ 379	$ 1,217	$ 1,125
Weighted-average remaining operating lease term (years)	4 years 7 months 6 days	1 year 10 months 24 days	4 years 7 months 6 days	1 year 10 months 24 days
Weighted-average discount rate	7.89%	8.61%	7.89%	8.61%