Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,166,679	$ 1,004,704
Direct premiums written	1,301,100	1,018,600
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	754,300	660,600
Direct premiums written	825,500	634,300
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	266,900	222,500
Direct premiums written	302,100	241,600
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	89,200	69,100
Direct premiums written	104,000	80,000
Other
Segment Reporting Information [Line Items]
Net premiums earned	56,300	52,500
Direct premiums written	69,500	62,700
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,159,369	997,562
Direct premiums written	1,294,475	1,012,238
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	754,300	660,600
Direct premiums written	825,500	634,300
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	266,900	222,500
Direct premiums written	302,100	241,600
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	89,200	69,100
Direct premiums written	104,000	80,000
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	49,000	45,400
Direct premiums written	62,900	56,300
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,100
Direct premiums written	6,600	6,400
Other | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Direct premiums written	0	0
Other | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Direct premiums written	0	0
Other | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Direct premiums written	0	0
Other | Other
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,100
Direct premiums written	$ 6,600	$ 6,400