Loans and allowance for loan losses - Additional information (Details) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Mar. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses, after ASU 2016-13				$ 150,000
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest	$ 3,681,000	$ 3,256,000	$ 3,272,000	3,681,000	$ 3,256,000	$ 3,423,000	$ 3,370,000	$ 3,574,000
Total Allowance for credit losses	4,000,000.0	3,530,000		4,000,000.0	3,530,000
Total Allowance for credit losses	3,681,000	3,256,000	$ 3,272,000	3,681,000	3,256,000	$ 3,423,000	$ 3,370,000	$ 3,574,000
Allowance for credit losses on unfunded credit exposure	320,000	277,000		320,000	277,000
Interest on non accrual loans				12,000
Provision for credit losses for loans	0	$ (22,000)		$ 136,700	$ 1,000
Loan growth being offset by improved credit metrics as non-performing loans as a percentage of loans		0.06%	0.12%	0.06%	0.06%	0.05%	0.13%
Net recoveries for credit losses for loans				$ 107,000
Provision for credit losses for unfunded commitments	0	$ 22,000		13,300	$ (1,000)
Asset Pledged as Collateral [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans pledged as collateral	$ 57,200,000			$ 57,200,000		$ 35,500,000
Guaranteed student loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage Of Recovery Loss On Portfolio From Historical Experience	98.00%			98.00%