Income taxes - Reconciliation of federal tax rate to the effective rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income taxes
U.S. statutory federal rate	21.00%	21.00%
State taxes, net of federal effect	4.00%	4.38%
Nontaxable foreign income	(17.28%)	(19.98%)
Research & Development Tax Credit	(11.73%)	(3.82%)
Permanent items	(0.67%)	(0.87%)
Valuation allowance for deferred tax assets	5.60%	(0.18%)
Effective Income Tax Rate Reconciliation, Percent	0.92%	0.53%