Income taxes - Narrative (Details) - USD ($) $ in Millions			12 Months Ended
	Jan. 01, 2020	Jan. 01, 2019	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2018
Income taxes
Income tax expense			$ 0.0	$ 0.0	$ 0.0
Valuation allowance increase due to increase in the deferred tax assets			$ 1.5	$ 8.6
Corporate income tax rate			21.00%	21.00%	21.00%
Uncertain tax positions			$ 0.1	$ 0.1	$ 0.8
Costs deemed direct or incidental to research and development capitalized			8.6
Foreign
Income taxes
Operating loss carryforwards			117.2
Federal
Income taxes
Operating loss carryforwards			53.8
State
Income taxes
Operating loss carryforwards			$ 53.5
Sweden.
Income taxes
Corporate income tax rate	20.60%	21.40%				22.00%