Income taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income taxes
Income tax expense	$ 0.0	$ 0.0	$ 0.0
Valuation allowance increase due to increase in the deferred tax assets	5.8	1.5
Uncertain tax positions	0.1	0.1
Costs deemed direct or incidental to research and development capitalized	7.8	$ 9.0
Foreign
Income taxes
Operating loss carryforwards	118.2
Federal
Income taxes
Operating loss carryforwards	65.5
State
Income taxes
Operating loss carryforwards	$ 66.3