Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net investment income	$ 52,443	$ 40,446	$ 141,227	$ 119,295
Net realized and unrealized investment (losses) gains	(4,787)	6,798	(16,988)	7,487
Total investment income, before federal income tax	47,656	47,244	124,239	126,782
Tax on investment income	8,562	13,236	21,405	34,572
Total investment income, after federal income tax	39,094	34,008	102,834	92,210
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	26,333	35,329	74,153	112,634
Underwriting gain (loss), after federal income tax	$ 20,803	$ 22,964	$ 58,581	$ 73,212
Combined ratio	94.50%	92.10%	94.80%	91.50%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 3,158	$ 8,179	$ 6,457	$ 7,517
Underwriting gain (loss), after federal income tax	$ 2,495	$ 5,316	$ 5,101	$ 4,886
Combined ratio	95.90%	88.70%	97.20%	96.50%
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 3,506	$ (11,063)	$ (4,848)	$ (8,174)
Underwriting gain (loss), after federal income tax	$ 2,770	$ (7,191)	$ (3,830)	$ (5,313)
Combined ratio	93.70%	120.40%	103.00%	105.20%

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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, after-tax

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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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After-tax amount of net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses.

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Net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses.

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Ratio of non-life losses and loss expenses, non-life acquisition expenses and non-life general and administrative expenses, to non-life net premiums earned.

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Amount of realized and unrealized gain (loss) on investment.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 320
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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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