Employee benefit plans - Salaries and Employee Benefits Expense - SERP and Employee Pension Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jun. 22, 2008
Supplemental Executive Retirement Plans ("SERP")
Defined Benefit Plan Disclosure
Maximum annual benefit in excess of federal limits (as a percent)	6.00%
Employee benefit plan liability	$ 180
Employee benefit plan expense	19	$ 20	$ 19
Defined benefit plan accrued liabilities	3,100	2,800
Compensation expense	$ 400	400	$ 400
Pension Benefits
Defined Benefit Plan Disclosure
Retirement age period for fixed benefits payable	65 years
Benefits accrued under employees' pension plan				$ 0
Increase (decrease) in accumulated benefit obligation	$ 300	$ (3,900)
Discount rate, Obligation	4.73%	4.93%	2.59%