Income taxes - Carryforwards, unrecognized benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Taxes [Line Items]
Increase in valuation allowance	$ 21,700,000	$ 32,000,000	$ 28,500,000
Operating loss carryforwards foreign	2,700,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning balance	1,272,000	1,003,000	670,000
Additions based on tax positions related to the current year	312,000	230,000	161,000
Additions based on tax positions related to prior years	52,000	39,000	172,000
Unrecognized tax benefits, ending balance	1,636,000	1,272,000	1,003,000
Unrecognized tax benefits, uncertainty in income taxes	1,800,000	1,400,000
Income tax expense related to interest and penalties	0	$ 0	$ 0
Domestic Tax Authority
Income Taxes [Line Items]
Operating loss carryforwards	125,900,000
Operating loss carryforwards subject to expiration	69,300,000
Operating loss carryforwards not subject to expiration	56,600,000
Domestic Tax Authority | Research Tax Credit Carryforward
Income Taxes [Line Items]
Tax credit carryforward, amount	11,000,000
Foreign Tax Authority
Income Taxes [Line Items]
Operating loss carryforwards	86,700,000
Tax credit carryforward, amount	1,400,000
Foreign Tax Authority | Research Tax Credit Carryforward
Income Taxes [Line Items]
Tax credit carryforward, amount	600,000
State and Local Jurisdiction
Income Taxes [Line Items]
Operating loss carryforwards	93,400,000
State and Local Jurisdiction | Research Tax Credit Carryforward
Income Taxes [Line Items]
Tax credit carryforward, amount	$ 4,100,000