Income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of income tax expense
Current federal income tax expense	$ 12,860	$ 9,923	$ 12,555
Deferred federal income tax benefit	(125)	(3,230)	1,065
Income tax expense (benefit)	12,735	6,693	13,620
Federal income taxes paid	11,600	9,900	$ 13,500
Income tax payable	$ 400
Income taxes receivable		$ 800
Statutory income tax rate, percent	21.00%	21.00%	35.00%
Effective income tax rate reconciliation, amount [Abstract]
Income tax expense at federal income tax	$ 15,971	$ 8,501	$ 13,482
Stock options exercised	(2,411)	(918)	471
Tax-exempt investment income	(577)	(672)	(1,064)
Effect of tax rate change	0	0	1,915
Other	(248)	(218)	(242)
Income tax expense (benefit)	12,735	6,693	13,620
Unrecognized tax positions	$ 0	$ 0
Decrease in unrecognized tax positions			$ 1,000