Income taxes - Reconciliation of federal tax rate to the effective benefit rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income taxes
U.S. statutory federal rate	21.00%	21.00%
State taxes, net of federal effect	4.38%	4.22%
Nontaxable foreign income	(30.09%)	(38.26%)
Research & Development Tax Credit	(3.46%)	(2.27%)
Permanent items	(0.79%)	1.26%
Valuation allowance for deferred tax assets	9.45%	12.72%
Effective Income Tax Rate Reconciliation, Percent	0.49%	(1.33%)