Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,320,652	$ 1,090,311	$ 3,723,355	$ 3,129,483
Direct premiums written	1,465,700	1,237,200	4,154,100	3,395,200
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	854,400	713,200	2,408,300	2,048,500
Direct premiums written	878,200	772,800	2,551,300	2,105,600
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	301,800	244,000	852,300	700,400
Direct premiums written	370,200	307,700	1,034,200	843,800
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	97,300	78,400	280,700	219,600
Direct premiums written	95,800	93,700	298,600	256,400
Other
Segment Reporting Information [Line Items]
Net premiums earned	67,200	54,700	182,100	161,000
Direct premiums written	121,500	63,000	270,000	189,400
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,313,282	1,082,979	3,701,347	3,107,812
Direct premiums written	1,458,934	1,230,024	4,133,920	3,374,477
Property & Casualty \| Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	854,400	713,200	2,408,300	2,048,500
Direct premiums written	878,200	772,800	2,551,300	2,105,600
Property & Casualty \| Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	301,800	244,000	852,300	700,400
Direct premiums written	370,200	307,700	1,034,200	843,800
Property & Casualty \| Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	97,300	78,400	280,700	219,600
Direct premiums written	95,800	93,700	298,600	256,400
Property & Casualty \| Other
Segment Reporting Information [Line Items]
Net premiums earned	59,800	47,400	160,000	139,300
Direct premiums written	114,700	55,800	249,800	168,700
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,400	7,300	22,100	21,700
Direct premiums written	6,800	7,200	20,200	20,700
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	7,400	7,300	22,100	21,700
Direct premiums written	$ 6,800	$ 7,200	$ 20,200	$ 20,700