Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 996,939	$ 940,941	$ 2,947,000	$ 2,783,682
Direct premiums written	1,057,100	1,032,100	3,109,500	2,958,400
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	665,600	652,000	1,990,900	1,956,600
Direct premiums written	680,600	693,900	2,015,600	2,006,400
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	213,000	178,900	610,900	509,100
Direct premiums written	251,800	222,600	713,900	600,700
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	68,000	65,600	198,600	192,800
Direct premiums written	66,500	62,300	207,900	197,400
Other
Segment Reporting Information [Line Items]
Net premiums earned	50,300	44,400	146,600	125,200
Direct premiums written	58,200	53,300	172,100	153,900
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	989,688	933,605	2,925,410	2,761,934
Direct premiums written	1,049,088	1,024,753	3,086,222	2,934,777
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	665,600	652,000	1,990,900	1,956,600
Direct premiums written	680,600	693,900	2,015,600	2,006,400
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	213,000	178,900	610,900	509,100
Direct premiums written	251,800	222,600	713,900	600,700
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	68,000	65,600	198,600	192,800
Direct premiums written	66,500	62,300	207,900	197,400
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	43,100	37,100	125,000	103,400
Direct premiums written	50,200	46,000	148,800	130,300
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,300	21,600	21,800
Direct premiums written	8,000	7,300	23,300	23,600
Other | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other | Other
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,300	21,600	21,800
Direct premiums written	$ 8,000	$ 7,300	$ 23,300	$ 23,600