Income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Income tax expense	$ 0	$ 0
Valuation allowance	3,700	3,400
Federal operating loss	47,300	35,300
State net operating loss	47,700	35,300
Net operating loss generated not subject to expiration		18,300	$ 18,300
Uncertain tax positions	$ 0	0
Percentage of taxable income offset from net operating loss	80.00%
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss generated subject to expiration	$ 12,300	$ 10,000
R&D tax credits	$ 1,400