Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,004,704	$ 962,550
Direct premiums written	1,018,600	1,018,200
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	660,600	659,300
Direct premiums written	634,300	681,300
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	222,500	192,000
Direct premiums written	241,600	210,200
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	69,100	64,500
Direct premiums written	80,000	71,600
Other
Segment Reporting Information [Line Items]
Net premiums earned	52,500	46,800
Direct premiums written	62,700	55,100
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	997,562	955,418
Direct premiums written	1,012,238	1,011,385
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	660,600	659,300
Direct premiums written	634,300	681,300
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	222,500	192,000
Direct premiums written	241,600	210,200
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	69,100	64,500
Direct premiums written	80,000	71,600
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	45,400	39,600
Direct premiums written	56,300	48,300
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,200
Direct premiums written	6,400	6,800
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	7,100	7,200
Direct premiums written	$ 6,400	$ 6,800