Income taxes (Details 2)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
U.S. statutory federal rate	21.00%	34.00%
State taxes, net of federal effect	4.22%	2.00%
Foreign tax rate differential	(38.26%)	(82.91%)
R&D tax credit	(2.27%)	(2.49%)
Permanent items	1.26%	13.39%
Tax Act adjustment	0.00%	(11.25%)
Valuation allowance for deferred tax assets	12.72%	28.90%
Effective Income Tax Rate Reconciliation, Percent	(1.33%)	(18.36%)