Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 3,952,482	$ 3,741,948	$ 3,555,635
Direct premiums written	4,048,500	3,907,500	3,654,200
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	2,655,700	2,613,500	2,591,300
Direct premiums written	2,598,900	2,640,700	2,569,000
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	830,600	698,100	601,300
Direct premiums written	940,300	801,500	678,900
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	267,900	258,600	225,600
Direct premiums written	276,800	259,900	240,800
Other
Segment Reporting Information [Line Items]
Net premiums earned	198,300	171,700	137,400
Direct premiums written	232,500	205,400	165,500
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	3,923,642	3,712,910	3,526,753
Direct premiums written	4,018,057	3,877,476	3,624,965
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	2,655,700	2,613,500	2,591,300
Direct premiums written	2,598,900	2,640,700	2,569,000
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	830,600	698,100	601,300
Direct premiums written	940,300	801,500	678,900
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	267,900	258,600	225,600
Direct premiums written	276,800	259,900	240,800
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	169,400	142,700	108,600
Direct premiums written	202,100	175,400	136,300
Other
Segment Reporting Information [Line Items]
Net premiums earned	28,900	29,000	28,800
Direct premiums written	30,400	30,000	29,200
Other | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Other
Segment Reporting Information [Line Items]
Net premiums earned	28,900	29,000	28,800
Direct premiums written	$ 30,400	$ 30,000	$ 29,200