Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount recognized for uncertainty in income taxes classified as noncurrent.

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Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

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Income taxes - Narrative (Details) - USD ($) $ in Millions			12 Months Ended
Income taxes - Narrative (Details) - USD ($) $ in Millions	Jan. 01, 2020	Jan. 01, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	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			8.6		13.9
Uncertain tax positions			$ 0.1	$ 0.8	$ 0.0
Corporate income tax rate			21.00%	21.00%	21.00%
Foreign
Income taxes
Operating loss carryforwards			$ 126.9
Domestic
Income taxes
Operating loss carryforwards			45.6
State
Income taxes
Operating loss carryforwards			$ 45.2
Sweden.
Income taxes
Corporate income tax rate	20.60%	21.40%				22.00%