Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred tax liabilities:
Unrealized gain on fixed income and equity security investments	$ 19,098	$ 14,319
Deferred acquisition costs	2,782	3,576
Salvage and subrogation	2,555	2,275
Limited partnership investments	1,554	0
Other	1,562	898
Total deferred tax liabilities	27,551	21,068
Deferred tax assets:
Loss and loss expense reserves	11,384	18,377
Unearned premiums discount	2,097	2,361
Other-than-temporary investment declines	2,733	2,902
Deferred ceding commission	1,700	1,974
Deferred compensation	734	845
Limited partnership investments	0	1,459
Other	325	269
Total deferred tax assets	18,973	28,187
Net Deferred tax assets (liabilities)	(8,578)	7,119
Summary of the Difference Between Federal Income Tax Expense computed at Statutory Rate [Abstract]
Statutory federal income rate applied to pretax income (loss)	16,312	(15,851)	12,185
Tax effect of (deduction):
Tax-exempt investment income	(933)	(1,145)	(1,296)
Net reduction of tax positions	(693)	(174)	(826)
Other	1	56	(265)
Federal income tax expense (benefit)	14,687	(17,114)	9,798
Taxes (credits) on pre-tax income (loss) [Abstract]
Current	10,819	(8,205)	9,752
Deferred	3,868	(8,909)	46
Federal income tax expense (benefit)	14,687	(17,114)	9,798
Components of the provisions for deferred federal income taxes (credits) [Abstract]
Limited partnerships	3,013	(7,196)	485
Discounts of loss and loss expense reserves	(57)	(2,546)	(107)
Unearned premium discount	265	(30)	(517)
Deferred compensations	112	499	(735)
Other-than-temporary investment decline	169	(240)	571
Deferred acquisitions costs and ceding commission	(520)	(87)	(27)
Other	886	691	376
Provision for deferred federal income tax (credits)	3,868	(8,909)	46
Cash flows related to federal income taxes paid, net of refunds received	3,661	2,859	9,000
Reconciliation of the beginning and ending amounts of unrecognized federal income taxes (credits) [Roll Forward]
Balance at beginning of period	6,000	6,000
Reductions for tax positions of the current year	0	0
Reductions for tax positions of prior years	(6,000)	0
Balance at end of period	$ 0	$ 6,000	$ 6,000