Segment Information - Schedule of Reconciliation of Segment Results to Income (Loss) Before 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, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Investment income	$ 109,939	$ 92,270	$ 216,153	$ 187,120
Interest expense	(7,202)	(7,258)	(14,383)	(14,424)
Corporate expenses	(9,154)	(9,329)	(24,652)	(21,437)
Income (loss), before federal income tax	(80,098)	74,157	22,468	188,916
Preferred stock dividends	(2,300)	(2,300)	(4,600)	(4,600)
Income (loss) available to common stockholders, before federal income tax	(82,398)	71,857	17,868	184,316
Standard Commercial Lines
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting income (loss)	(160,881)	22,146	(150,500)	61,067
Standard Personal Lines
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting income (loss)	(19,301)	(23,060)	(24,636)	(36,133)
E&S Lines
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting income (loss)	6,501	(612)	20,486	12,723
Investments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Investment income	109,939	92,270	216,153	187,120
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total all segments	$ (63,742)	$ 90,744	$ 61,503	$ 224,777