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Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Investment income, before federal income tax	$ 36,374	$ 28,065
Interest expense	(6,106)	(5,606)
General corporate and other expenses	(11,916)	(11,539)
Income before federal income tax	67,574	51,875
Standard Commercial Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain, before federal income tax	42,546	30,932
Standard Personal Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain, before federal income tax	5,106	8,605
Excess And Surplus Operations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain, before federal income tax	1,570	1,418
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total all segments	$ 85,596	$ 69,020

X

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The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses.

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Total of all reportable segments' income or loss from continuing operations, before federal income taxes.

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Amount of general expenses not normally included in Other Operating Costs and Expenses.

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