Income taxes (Details 2)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
U.S. statutory federal rate	34.00%	34.00%
State taxes, net of federal effect	2.00%	(2.00%)
Foreign tax rate differential	(82.91%)	(128.30%)
R&D tax credit	(2.49%)	0.00%
Permanent items	13.39%	20.90%
Tax Act adjustment	(11.25%)	0.00%
Valuation allowance for deferred tax assets	28.90%	51.40%
Effective Income Tax Rate Reconciliation, Percent	(18.36%)	(24.00%)