Income taxes - Reconciliation of federal tax rate to the effective rate (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income taxes
U.S. statutory federal rate	21.00%	21.00%
State taxes, net of federal effect	2.88%	8.88%
Nontaxable foreign income	5.71%	25.57%
Research & Development Tax Credit		(1.92%)
Permanent items	(0.32%)	(0.70%)
Valuation allowance for deferred tax assets	(0.08%)	2.38%
Total U.S. statutory federal tax rate to the effective rate	17.77%	4.07%