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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, 2018	Mar. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Investment income, before federal income tax	$ 32,682	$ 36,374
Interest expense	(6,152)	(6,106)
Corporate expenses	(11,332)	(11,916)
Income before federal income tax	19,931	67,574
Standard Commercial Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax	6,804	42,546
Standard Personal Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax	(1,506)	5,106
Excess And Surplus Operations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax	(565)	1,570
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total all segments	$ 37,415	$ 85,596

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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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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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-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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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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