Income Taxes - Rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Rate reconciliation
Federal tax benefit at statutory rate	(21.00%)	(21.00%)	(21.00%)
State rate, net of federal benefit	(3.50%)	(4.20%)	(2.90%)
Permanent differences	0.2	0.6	0.4
Research and development	0.40%	0.70%	(0.70%)
Change in valuation allowance	24.20%	24.00%	24.20%
Other	(0.30%)	(0.10%)	0.00%
Total tax provision