Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) State	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment State Subsidiary	Sep. 30, 2022 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary			12
Number of states in which company operates | State	11		11
Number of reportable segments | segment			1
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,090,311	$ 996,939	$ 3,129,483	$ 2,947,000
Losses and loss adjustment expenses	823,742	787,462	2,651,081	2,436,175
Underwriting expenses	250,800	237,400	726,000	695,800
Underwriting gain (loss)	15,800	(28,000)	(247,600)	(185,000)
Net investment income	60,965	44,563	171,287	118,469
Net realized investment losses	(90,001)	(144,213)	(60,772)	(581,237)
Other income	3,917	2,998	15,000	7,141
Interest expense	(5,918)	(4,273)	(16,398)	(12,822)
Pre-tax loss	(15,229)	(128,903)	(138,517)	(653,373)
Net loss	(8,227)	(98,303)	(95,058)	(505,902)
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,082,979	989,688	3,107,812	2,925,410
Losses and loss adjustment expenses	819,700	783,800	2,639,400	2,425,100
Underwriting expenses	247,100	233,600	715,100	687,300
Underwriting gain (loss)	16,200	(27,700)	(246,700)	(187,000)
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,200	21,700	21,600
Losses and loss adjustment expenses	4,000	3,700	11,700	11,100
Underwriting expenses	3,700	3,800	10,900	8,500
Underwriting gain (loss)	$ (400)	$ (300)	$ (900)	$ 2,000