Employee benefit plans - Components of Net Periodic Pension Expense (Income) and Other Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Net loss (gain)	$ 4,370	$ 5,499
Pension Benefits
Components of net periodic pension cost:
Interest cost	655	456	$ 419
Expected return on plan assets	(1,170)	(1,407)	(1,288)
Amortization of unrecognized net loss	194	198	301
Net periodic pension income:	(321)	(753)	(568)
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Net loss (gain)	655	456	419
Deferred tax	(141)	(96)	(88)
Total recognized in other comprehensive income (loss)	514	360	331
Total recognized in net period pension cost and other comprehensive income (loss)	$ 193	$ (393)	$ (237)