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Reportable Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net premiums earned	$ 68,933us-gaap_PremiumsEarnedNet	$ 65,947us-gaap_PremiumsEarnedNet	$ 62,905us-gaap_PremiumsEarnedNet	$ 63,842us-gaap_PremiumsEarnedNet	$ 65,422us-gaap_PremiumsEarnedNet	$ 64,448us-gaap_PremiumsEarnedNet	$ 61,775us-gaap_PremiumsEarnedNet	$ 61,098us-gaap_PremiumsEarnedNet	$ 261,627us-gaap_PremiumsEarnedNet	$ 252,743us-gaap_PremiumsEarnedNet	$ 237,461us-gaap_PremiumsEarnedNet
Reconciliation from Segment Totals to Consolidated [Abstract]
Net investment income									9,055us-gaap_InvestmentIncomeNet	8,770us-gaap_InvestmentIncomeNet	9,930us-gaap_InvestmentIncomeNet
Income before federal income taxes									44,398	54,910	46,606
Reportable Segments [Member]
Segment Reporting Information [Line Items]
Direct and assumed premium written									382,388	369,476	341,286
Net premiums earned									261,627	252,743	237,461
Underwriting gain									34,810	37,836	44,058
Reconciliation from Segment Totals to Consolidated [Abstract]
Underwriting gain									34,810	37,836	44,058
Reportable Segments [Member] \| Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Direct and assumed premium written									343,200	314,784	284,200
Net premiums earned									224,683	202,785	186,586
Underwriting gain									32,663	25,558	23,491
Reconciliation from Segment Totals to Consolidated [Abstract]
Underwriting gain									32,663	25,558	23,491
Reportable Segments [Member] \| Reinsurance [Member]
Segment Reporting Information [Line Items]
Direct and assumed premium written									39,188	54,692	57,086
Net premiums earned									36,944	49,958	50,875
Underwriting gain									2,147	12,278	20,567
Reconciliation from Segment Totals to Consolidated [Abstract]
Underwriting gain									2,147	12,278	20,567
Segment Reconciling Items [Member]
Reconciliation from Segment Totals to Consolidated [Abstract]
Net investment income									9,055	8,770	9,930
Net realized gains on investments									14,930	23,515	9,011
Corporate expenses									$ (14,397)	$ (15,211)	$ (16,393)