Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($) Subsidiary segment State	Mar. 31, 2021 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary	13
Number of states in which company operates | State	11
Number of reportable segments | segment	1
Segment Reporting Information [Line Items]
Net premiums earned	$ 962,550	$ 915,922
Losses and loss adjustment expenses	821,933	626,344
Underwriting expenses	232,400	230,100
Underwriting gain (loss)	(91,700)	59,500
Net investment income	35,351	32,279
Net realized investment (losses) gains	(195,086)	41,691
Other income	2,646	3,204
Interest expense	(4,275)	(4,343)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(253,129)	132,421
Net (loss) income	(196,917)	106,995
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	955,418	908,815
Losses and loss adjustment expenses	818,300	622,900
Underwriting expenses	229,100	227,300
Underwriting gain (loss)	(92,000)	58,600
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,100
Losses and loss adjustment expenses	3,600	3,400
Underwriting expenses	3,300	2,800
Underwriting gain (loss)	$ 300	$ 900