Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Total investment segment income, after federal income tax	$ 10,197	$ 51,949
Net investment income	55,967	50,618
Net realized and unrealized investment (losses) gains	(44,666)	13,451
Total investment segment income, before federal income tax	11,301	64,069
Tax on investment income	1,104	12,120
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	17,126	25,815
Underwriting gain (loss), after federal income tax	$ 13,529	$ 20,394
Combined ratio	96.70%	94.80%
ROE Contribution	2.40%	4.40%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 387	$ 3,167
Underwriting gain (loss), after federal income tax	$ 306	$ 2,502
Combined ratio	99.50%	95.90%
ROE Contribution	0.10%	0.50%
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 3,844	$ 4,579
Underwriting gain (loss), after federal income tax	$ 3,037	$ 3,617
Combined ratio	93.50%	92.10%
ROE Contribution	0.60%	0.80%
Investments [Member]
Segment Reporting Information [Line Items]
ROE Contribution	8.50%	8.90%

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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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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