Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2022 USD ($) State Subsidiary segment	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary	12
Number of states in which company operates | State	11
Number of reportable segments | segment	1
Segment Reporting Information [Line Items]
Net premiums earned	$ 3,952,482	$ 3,741,948	$ 3,555,635
Losses and loss adjustment expenses	3,362,219	2,760,155	2,395,343
Underwriting expenses	934,400	916,700	913,700
Underwriting (loss) gain	(344,100)	65,000	246,600
Net investment income	168,356	129,727	134,858
Net realized investment (losses) gains	(488,080)	111,658	85,731
Other income	10,308	10,024	8,287
Interest expense	(17,232)	(17,113)	(17,048)
(Loss) income before income taxes	(670,715)	299,307	458,501
Net (loss) income	(512,672)	247,937	374,607
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	3,923,642	3,712,910	3,526,753
Losses and loss adjustment expenses	3,347,200	2,746,300	2,381,500
Underwriting expenses	922,100	902,600	899,100
Underwriting (loss) gain	(345,700)	64,000	246,200
Other
Segment Reporting Information [Line Items]
Net premiums earned	28,900	29,000	28,800
Losses and loss adjustment expenses	15,000	13,900	13,800
Underwriting expenses	12,300	14,100	14,600
Underwriting (loss) gain	$ 1,600	$ 1,000	$ 400