Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred tax liabilities:
Unrealized gain on fixed income and equity security investments	$ 27,914	$ 26,432
Deferred acquisition costs	1,341	1,829
Salvage and subrogation	2,373	2,638
Limited partnership investments	4,636	2,612
Other	1,229	866
Total deferred tax liabilities	37,493	34,377
Deferred tax assets:
Loss and loss expense reserves	10,973	11,071
Unearned premiums discount	2,201	2,162
Other-than-temporary investment declines	2,049	2,053
Deferred ceding commission	549	1,018
Deferred compensation	1,565	880
Other	183	342
Total deferred tax assets	17,520	17,526
Net deferred tax liabilities	(19,973)	(16,851)
Summary of the Difference Between Federal Income Tax Expense computed at Statutory Rate [Abstract]
Statutory federal income rate applied to pretax income	15,539	19,218	16,312
Tax effect of (deduction):
Tax-exempt investment income	(924)	(811)	(933)
Net reduction of tax positions	(75)	(116)	(693)
Other	141	31	1
Federal income tax expense	14,681	18,322	14,687
Taxes on pre-tax income [Abstract]
Current	13,041	17,383	10,819
Deferred	1,640	939	3,868
Federal income tax expense	14,681	18,322	14,687
Components of the provisions for deferred federal income taxes (credits) [Abstract]
Limited partnerships	2,025	1,058	3,013
Discounts of loss and loss expense reserves	113	313	(57)
Unearned premium discount	(38)	(65)	265
Deferred compensation	(685)	(146)	112
Other-than-temporary investment declines	(19)	680	169
Deferred acquisitions costs and ceding commission	(20)	(271)	(520)
Other	264	(630)	886
Provision for deferred federal income tax	1,640	939	3,868
Cash flows related to federal income taxes paid, net of refunds received	$ 11,619	$ 17,250	$ 3,661