Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 962,550	$ 915,922
Direct premiums written	1,018,200	953,400
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	659,300	653,700
Direct premiums written	681,300	666,500
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	192,000	160,800
Direct premiums written	210,200	170,200
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	64,500	62,600
Direct premiums written	71,600	68,900
Other
Segment Reporting Information [Line Items]
Net premiums earned	46,800	38,800
Direct premiums written	55,100	47,800
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	955,418	908,815
Direct premiums written	1,011,385	946,606
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	659,300	653,700
Direct premiums written	681,300	666,500
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	192,000	160,800
Direct premiums written	210,200	170,200
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	64,500	62,600
Direct premiums written	71,600	68,900
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	39,600	31,700
Direct premiums written	48,300	41,000
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,100
Direct premiums written	6,800	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	7,200	7,100
Direct premiums written	$ 6,800	$ 6,800