Income taxes (Details) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Income taxes
Income tax expense		$ 0	$ 0
Operating loss carryforwards which may be subject to limitations				$ 12,500,000
Uncertain tax positions		$ 0	$ 0	0
Sweden
Income taxes
Foreign income tax rate (as a percent)	20.60%
Reduction in valuation allowance	$ (700,000)
Reduction in deferred income taxes	$ 0
Sweden | Minimum
Income taxes
Foreign income tax rate (as a percent)	21.40%
Sweden | Maximum
Income taxes
Foreign income tax rate (as a percent)	22.00%
Foreign
Income taxes
Operating loss carryforwards				61,600,000
Federal
Income taxes
Operating loss carryforwards				500,000
State
Income taxes
Operating loss carryforwards				$ 400,000