Income Taxes - Rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Rate reconciliation
Federal tax benefit at statutory rate	(21.00%)	(21.00%)	(21.00%)
State rate, net of federal benefit	(3.10%)	(3.10%)	(3.50%)
Permanent differences	0.7	0.6	0.2
Research and development	(0.40%)		0.40%
Change in valuation allowance	23.40%	23.40%	24.20%
Other	0.40%	0.10%	(0.30%)
Total tax provision