Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,090,311	$ 996,939	$ 3,129,483	$ 2,947,000
Direct premiums written	1,237,200	1,057,100	3,395,200	3,109,500
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	713,200	665,600	2,048,500	1,990,900
Direct premiums written	772,800	680,600	2,105,600	2,015,600
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	244,000	213,000	700,400	610,900
Direct premiums written	307,700	251,800	843,800	713,900
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	78,400	68,000	219,600	198,600
Direct premiums written	93,700	66,500	256,400	207,900
Other
Segment Reporting Information [Line Items]
Net premiums earned	54,700	50,300	161,000	146,600
Direct premiums written	63,000	58,200	189,400	172,100
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,082,979	989,688	3,107,812	2,925,410
Direct premiums written	1,230,024	1,049,088	3,374,477	3,086,222
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	713,200	665,600	2,048,500	1,990,900
Direct premiums written	772,800	680,600	2,105,600	2,015,600
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	244,000	213,000	700,400	610,900
Direct premiums written	307,700	251,800	843,800	713,900
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	78,400	68,000	219,600	198,600
Direct premiums written	93,700	66,500	256,400	207,900
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	47,400	43,100	139,300	125,000
Direct premiums written	55,800	50,200	168,700	148,800
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,200	21,700	21,600
Direct premiums written	7,200	8,000	20,700	23,300
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	7,300	7,200	21,700	21,600
Direct premiums written	$ 7,200	$ 8,000	$ 20,700	$ 23,300