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Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Net investment income earned	$ 91,506	$ 72,602
Net realized and unrealized investment gains (losses)	3,344	(40,352)
Total investment segment income, before federal income tax	94,850	32,250
Tax on investments segment income	19,156	5,613
Total investment segment income, after federal income tax	75,694	26,637
Standard Commercial Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	38,921	42,384
Underwriting income (loss), after federal income tax	$ 30,748	$ 33,483
Combined ratio	94.70%	93.60%
ROE Contribution	5.10%	5.00%
Standard Personal Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	$ (13,073)	$ 6,520
Underwriting income (loss), after federal income tax	$ (10,328)	$ 5,151
Combined ratio	116.00%	91.00%
ROE Contribution	(1.70%)	0.80%
E&S Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	$ 13,335	$ 6,925
Underwriting income (loss), after federal income tax	$ 10,535	$ 5,471
Combined ratio	85.00%	91.10%
ROE Contribution	1.80%	0.80%
Investments
Segment Reporting Information [Line Items]
ROE Contribution	12.20%	8.70%

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

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