Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,034,469	$ 987,512	$ 2,039,173	$ 1,950,062
Direct premiums written	1,139,500	1,034,100	2,158,000	2,052,300
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	674,700	666,000	1,335,300	1,325,300
Direct premiums written	698,500	653,800	1,332,800	1,335,000
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	233,900	205,900	456,400	397,900
Direct premiums written	294,500	251,800	536,100	462,100
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	72,100	66,100	141,200	130,600
Direct premiums written	82,700	69,800	162,700	141,400
Other
Segment Reporting Information [Line Items]
Net premiums earned	53,800	49,500	106,300	96,300
Direct premiums written	63,800	58,700	126,400	113,800
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,027,270	980,304	2,024,832	1,935,722
Direct premiums written	1,132,216	1,025,749	2,144,453	2,037,134
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	674,700	666,000	1,335,300	1,325,300
Direct premiums written	698,500	653,800	1,332,800	1,335,000
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	233,900	205,900	456,400	397,900
Direct premiums written	294,500	251,800	536,100	462,100
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	72,100	66,100	141,200	130,600
Direct premiums written	82,700	69,800	162,700	141,400
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	46,600	42,300	91,900	81,900
Direct premiums written	56,500	50,300	112,900	98,600
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,200	14,400	14,400
Direct premiums written	7,300	8,400	13,500	15,200
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	7,200	7,200	14,400	14,400
Direct premiums written	$ 7,300	$ 8,400	$ 13,500	$ 15,200