Income taxes - Narrative (Details) - USD ($)			1 Months Ended			12 Months Ended
	Jan. 01, 2020	Jan. 01, 2019	Jun. 30, 2018	May 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income tax expense						$ 0	$ 0	$ 0
Valuation allowance increase due to increase in the deferred tax assets						7,000,000	2,500,000
Uncertain tax positions						0
Recognized interest and penalties related to income tax matters as a component of income tax expense						$ 0	$ 0	$ 0
Corporate income tax rate					35.00%	21.00%	21.00%	34.00%
Increase (decrease) in deferred income taxes			$ (700,000)			$ (100,000)
Foreign
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards						89,900,000
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards						7,300,000
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards						$ 5,800,000
Swedish income tax
Operating Loss Carryforwards [Line Items]
Corporate income tax rate	20.60%	21.40%		22.00%