Bank call loans - Schedule of Bank Call Loans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Bank Call Loans [Abstract]
Year-end balance	$ 0	$ 0
Weighted interest rate (at end of year) (as a percent)	0.00%	0.00%
Maximum balance (at any month-end)	$ 131,900,000	$ 179,200,000
Average amount outstanding (during the year)	$ 49,387,000	$ 79,366,000
Average interest rate (during the year) (as a percent)	4.20%	2.18%