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Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Net investment income	$ 72,602	$ 69,716
Net realized and unrealized investment (losses) gains	(40,352)	5,119
Total investment segment income, before federal income tax	32,250	74,835
Tax on investment income	5,613	14,448
Total investment segment income, after federal income tax	26,637	60,387
Standard Commercial Lines
Segment Reporting Information [Line Items]
Underwriting income, before federal income tax	42,384	69,499
Underwriting income, after federal income tax	$ 33,483	$ 54,904
Combined ratio	93.60%	88.20%
ROE Contribution	5.00%	8.60%
Standard Personal Lines
Segment Reporting Information [Line Items]
Underwriting income, before federal income tax	$ 6,520	$ 7,695
Underwriting income, after federal income tax	$ 5,151	$ 6,079
Combined ratio	91.00%	89.60%
ROE Contribution	0.80%	1.00%
E&S Lines
Segment Reporting Information [Line Items]
Underwriting income, before federal income tax	$ 6,925	$ 516
Underwriting income, after federal income tax	$ 5,471	$ 408
Combined ratio	91.10%	99.20%
ROE Contribution	0.80%	0.10%
Investments
Segment Reporting Information [Line Items]
ROE Contribution	8.70%	8.90%

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

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