Segment Information - Schedule of Income (Loss) Before and After Federal Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Net investment income earned	$ 108,642	$ 97,696	$ 216,491	$ 189,202
Net realized and unrealized investment gains (losses)	1,297	(5,426)	(338)	(2,082)
Total investments segment income, before federal income tax	109,939	92,270	216,153	187,120
Tax on investments segment income	22,652	18,745	44,518	37,901
Total investments segment income, after federal income tax	87,287	73,525	171,635	149,219
Standard Commercial Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	(160,881)	22,146	(150,500)	61,067
Underwriting income (loss), after federal income tax	$ (127,096)	$ 17,495	$ (118,895)	$ 48,243
Combined ratio	118.80%	97.10%	108.90%	95.90%
ROE contribution	(18.40%)	2.80%	(8.60%)	4.10%
Standard Personal Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	$ (19,301)	$ (23,060)	$ (24,636)	$ (36,133)
Underwriting income (loss), after federal income tax	$ (15,248)	$ (18,217)	$ (19,462)	$ (28,545)
Combined ratio	118.10%	126.50%	111.70%	121.40%
ROE contribution	(2.20%)	(2.90%)	(1.40%)	(2.40%)
E&S Lines
Segment Reporting Information [Line Items]
Underwriting income (loss), before federal income tax	$ 6,501	$ (612)	$ 20,486	$ 12,723
Underwriting income (loss), after federal income tax	$ 5,136	$ (483)	$ 16,184	$ 10,051
Combined ratio	94.60%	100.70%	91.20%	93.00%
ROE contribution	0.70%	(0.10%)	1.20%	0.80%
Investments
Segment Reporting Information [Line Items]
ROE contribution	12.50%	12.60%	12.50%	12.50%
Total investments segment income, before federal income tax	$ 109,939	$ 92,270	$ 216,153	$ 187,120