Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Net investment income	$ 34,444	$ 58,505	$ 90,411	$ 109,123
Net realized and unrealized investment gains (losses)	12,649	4,027	(32,017)	17,478
Total investment segment income, before federal income tax	47,093	62,532	58,394	126,601
Tax on investment income	8,558	11,729	9,662	23,849
Total investment segment income, after federal income tax	38,535	50,803	48,732	102,752
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	16,730	37,143	33,856	62,958
Underwriting gain (loss), after federal income tax	$ 13,217	$ 29,343	$ 26,747	$ 49,737
Combined ratio	96.70%	92.70%	96.70%	93.70%
ROE Contribution	2.40%	5.90%	2.40%	5.20%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ (6,325)	$ 4,538	$ (5,938)	$ 7,705
Underwriting gain (loss), after federal income tax	$ (4,997)	$ 3,585	$ (4,691)	$ 6,087
Combined ratio	108.80%	94.10%	104.00%	95.00%
ROE Contribution	(0.90%)	0.70%	(0.40%)	0.60%
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ (532)	$ 2,944	$ 3,312	$ 7,523
Underwriting gain (loss), after federal income tax	$ (420)	$ 2,326	$ 2,616	$ 5,943
Combined ratio	100.90%	95.00%	97.20%	93.60%
ROE Contribution	(0.10%)	0.50%	0.20%	0.60%
Investments [Member]
Segment Reporting Information [Line Items]
ROE Contribution	5.20%	9.60%	6.60%	9.20%

The after-tax income divided by average stockholders' equity.

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Taxes incurred on investment income

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

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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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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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