Income taxes (Details) - USD ($)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income taxes paid		$ 13,500,000	$ 13,300,000	$ 13,600,000
Income taxes receivable		$ 800,000
Income taxes payable			100,000
Statutory income tax rate, percent	21.00%	35.00%
Transition adjustment for loss discounting		$ 3,524,000	0
Components of income tax expense
Current federal income tax expense		12,555,000	12,808,000	12,163,000
Deferred federal income tax benefit		1,065,000	561,000	(879,000)
Income tax expense (benefit)		13,620,000	13,369,000	11,284,000
Effective income tax rate reconciliation, amount [Abstract]
Income tax expense at federal income tax rate of 35%		13,482,000	13,837,000	11,745,000
Tax-exempt investment income		(1,064,000)	(528,000)	(436,000)
Stock options exercised		(471,000)	0	0
Effect of tax rate change		1,915,000	0	0
Other		(242,000)	60,000	(25,000)
Income tax expense (benefit)		$ 13,620,000	$ 13,369,000	$ 11,284,000