Bank call loans - Summary of Bank Call Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Bank Call Loans [Abstract]
Year-end balance	$ 69,500	$ 82,000
Weighted interest rate (at end of year) (as a percent)	0.96%	1.09%
Maximum balance (at any month-end)	$ 117,800	$ 203,100
Average amount outstanding (during the year)	$ 76,412	$ 82,760
Average interest rate (during the year) (as a percent)	0.95%	0.93%