Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Net investment income	$ 58,505	$ 45,553	$ 109,123	$ 88,784
Net realized and unrealized investment gains (losses)	4,027	(1,652)	17,478	(12,201)
Total investment income, before federal income tax	62,532	43,901	126,601	76,583
Tax on investment income	11,729	7,617	23,849	12,843
Total investment income, after federal income tax	50,803	36,284	102,752	63,740
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	37,143	41,016	62,958	47,820
Underwriting gain (loss), after federal income tax	$ 29,343	$ 32,403	$ 49,737	$ 37,778
Combined ratio	92.70%	91.40%	93.70%	94.90%
ROE Contribution	5.90%	7.80%	5.20%	4.50%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 4,538	$ 4,805	$ 7,705	$ 3,299
Underwriting gain (loss), after federal income tax	$ 3,585	$ 3,796	$ 6,087	$ 2,606
Combined ratio	94.10%	93.70%	95.00%	97.80%
ROE Contribution	0.70%	0.90%	0.60%	0.30%
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 2,944	$ (7,789)	$ 7,523	$ (8,354)
Underwriting gain (loss), after federal income tax	$ 2,326	$ (6,154)	$ 5,943	$ (6,600)
Combined ratio	95.00%	114.70%	93.60%	107.90%
ROE Contribution	0.50%	(1.50%)	0.60%	(0.80%)
Investments [Member]
Segment Reporting Information [Line Items]
ROE Contribution	9.60%	9.00%	9.20%	8.60%

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 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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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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