Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax liabilities [Abstract]
Unrealized gain on fixed income and equity security investments	$ 18,335	$ 20,959
Deferred acquisition costs	874	877
Loss and loss expense reserves	1,198	2,305
Limited partnership investments	2,274	1,771
Accelerated depreciation	1,037	897
Other	1,251	1,024
Total deferred tax liabilities	24,969	27,833
Deferred tax assets [Abstract]
Loss and loss expense reserves	9,467	9,349
Unearned premiums discount	1,295	1,593
Other-than-temporary investment declines	858	3,437
Deferred compensation	1,097	1,699
Deferred ceding commission	464	371
Other	376	186
Total deferred tax assets	13,557	16,635
Net deferred tax liabilities	(11,412)	(11,198)
Summary of the difference between federal income tax expense computed at statutory rate [Abstract]
Statutory federal income rate applied to pretax income	15,069	11,883	$ 15,539
Tax effect of (deduction) [Abstract]
Tax-exempt investment income	(938)	(919)	(924)
Other	(22)	(295)
Other			66
Federal income tax expense	14,109	10,669	14,681
Taxes (benefit) on pre-tax income [Abstract]
Current	11,271	12,488	13,041
Deferred	2,838	(1,819)	1,640
Federal income tax expense	14,109	10,669	14,681
Components of the provisions for deferred federal income taxes (credits) [Abstract]
Limited partnerships	503	(2,865)	2,025
Discounts of loss and loss expense reserves	(114)	1,526	113
Reserves - salvage and subrogation and other	(1,110)	29	(280)
Unearned premium discount	298	608	(38)
Deferred compensation	595	(127)	(685)
Other-than-temporary investment declines	2,320	(1,416)	(19)
Deferred acquisitions costs and ceding commission	(95)	(287)	(20)
Other	441	713	544
Provision for deferred federal income tax	2,838	(1,819)	1,640
Cash flows related to federal income taxes paid, net of refunds received	$ 10,173	$ 14,500	$ 11,619