Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Net investment income earned	$ 97,696	$ 70,222	$ 189,202	$ 142,824
Net realized and unrealized investment gains (losses)	(5,426)	(42,880)	(2,082)	(83,232)
Total investments segment income, before federal income tax	92,270	27,342	187,120	59,592
Tax on investments segment income	18,745	4,559	37,901	10,172
Total investments segment income, after federal income tax	73,525	22,783	149,219	49,420
Standard Commercial Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	22,146	46,708	61,067	89,092
Underwriting income (loss), after federal income tax	$ 17,495	$ 36,899	$ 48,243	$ 70,383
Combined ratio	97.10%	93.10%	95.90%	93.40%
ROE contribution	2.80%	6.00%	4.10%	5.50%
Standard Personal Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	$ (23,060)	$ (12,367)	$ (36,133)	$ (5,847)
Underwriting income (loss), after federal income tax	$ (18,217)	$ (9,770)	$ (28,545)	$ (4,619)
Combined ratio	126.50%	116.90%	121.40%	104.00%
ROE contribution	(2.90%)	(1.60%)	(2.40%)	(0.40%)
E&S Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	$ (612)	$ 3,372	$ 12,723	$ 10,297
Underwriting income (loss), after federal income tax	$ (483)	$ 2,664	$ 10,051	$ 8,135
Combined ratio	100.70%	95.80%	93.00%	93.50%
ROE contribution	(0.10%)	0.40%	0.80%	0.60%
Investments
Segment Reporting Information [Line Items]
ROE contribution	12.60%	9.10%	12.50%	8.90%