Income taxes (Details) - USD ($)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of income tax expense
Current federal income tax expense		$ 9,923,000	$ 12,555,000	$ 12,808,000
Deferred federal income tax benefit		(3,230,000)	1,065,000	561,000
Income tax expense (benefit)		6,693,000	13,620,000	13,369,000
Federal income taxes paid		9,900,000	13,500,000	13,300,000
Income taxes receivable		$ 800,000
Statutory income tax rate, percent	21.00%	35.00%
Transition adjustment for loss discounting		$ 2,060,000	3,524,000
Transition adjustment for loss discounting decrease		1,100,000
Effective income tax rate reconciliation, amount [Abstract]
Income tax expense at federal income tax		8,501,000	13,482,000	13,837,000
Stock options exercised		(918,000)	(471,000)	0
Tax-exempt investment income		(672,000)	(1,064,000)	(528,000)
Effect of tax rate change		0	1,915,000	0
Other		(218,000)	(242,000)	60,000
Income tax expense (benefit)		$ 6,693,000	$ 13,620,000	$ 13,369,000