Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) State Subsidiary segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 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	$ 3,741,948	$ 3,555,635	$ 3,599,418
Losses and loss adjustment expenses	2,760,155	2,395,343	2,706,024
Underwriting expenses	916,700	913,700	871,400
Underwriting gain	65,000	246,600	22,000
Net investment income	129,727	134,858	141,263
Net realized investment gains	111,658	85,731	222,793
Other income	10,024	8,287	9,044
Interest expense	(17,113)	(17,048)	(17,035)
Income before income taxes	299,307	458,501	378,069
Net income	247,937	374,607	320,087
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	3,712,910	3,526,753	3,571,042
Losses and loss adjustment expenses	2,746,300	2,381,500	2,692,700
Underwriting expenses	902,600	899,100	857,300
Underwriting gain	64,000	246,200	21,000
Other
Segment Reporting Information [Line Items]
Net premiums earned	29,000	28,800	28,400
Losses and loss adjustment expenses	13,900	13,800	13,300
Underwriting expenses	14,100	14,600	14,100
Underwriting gain	$ 1,000	$ 400	$ 1,000