Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net investment income	$ 55,826	$ 52,443	$ 164,949	$ 141,227
Net realized and unrealized investment (losses) gains	(2,183)	(4,787)	15,295	(16,988)
Total investment income, before federal income tax	53,643	47,656	180,244	124,239
Tax on investment income	10,884	8,562	34,733	21,405
Total investment income, after federal income tax	42,759	39,094	145,511	102,834
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	30,016	26,333	92,974	74,153
Underwriting gain (loss), after federal income tax	$ 23,713	$ 20,803	$ 73,450	$ 58,581
Combined ratio	94.20%	94.50%	93.90%	94.80%
ROE Contribution	4.50%	4.90%	5.00%	4.50%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ (631)	$ 3,158	$ 7,074	$ 6,457
Underwriting gain (loss), after federal income tax	$ (499)	$ 2,495	$ 5,588	$ 5,101
Combined ratio	100.80%	95.90%	96.90%	97.20%
ROE Contribution	(0.10%)	0.60%	0.40%	0.40%
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 1,916	$ 3,506	$ 9,439	$ (4,848)
Underwriting gain (loss), after federal income tax	$ 1,514	$ 2,770	$ 7,457	$ (3,830)
Combined ratio	96.90%	93.70%	94.70%	103.00%
ROE Contribution	0.30%	0.60%	0.50%	(0.30%)
Investments [Member]
Segment Reporting Information [Line Items]
ROE Contribution	8.60%	10.00%	9.10%	9.00%

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.

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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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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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