Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,410,400	$ 1,320,652	$ 4,060,208	$ 3,723,355
Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	1,410,400	1,320,700	4,060,200	3,723,400
Direct premiums written	1,561,700	1,465,700	4,492,200	4,154,100
Private passenger automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	900,000	854,400	2,646,000	2,408,300
Direct premiums written	919,900	878,200	2,707,900	2,551,300
Homeowners \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	334,200	301,800	907,100	852,300
Direct premiums written	446,600	370,200	1,225,300	1,034,200
Commercial automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	96,100	97,300	287,300	280,700
Direct premiums written	91,800	95,800	292,200	298,600
Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	80,100	67,200	219,800	182,100
Direct premiums written	103,400	121,500	266,800	270,000
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,402,961	1,313,282	4,038,061	3,701,347
Direct premiums written	1,554,908	1,458,934	4,470,544	4,133,920
Property & Casualty \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	1,403,000	1,313,300	4,038,100	3,701,300
Direct premiums written	1,554,900	1,458,900	4,470,500	4,133,900
Property & Casualty \| Private passenger automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	900,000	854,400	2,646,000	2,408,300
Direct premiums written	919,900	878,200	2,707,900	2,551,300
Property & Casualty \| Homeowners \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	334,200	301,800	907,100	852,300
Direct premiums written	446,600	370,200	1,225,300	1,034,200
Property & Casualty \| Commercial automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	96,100	97,300	287,300	280,700
Direct premiums written	91,800	95,800	292,200	298,600
Property & Casualty \| Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	72,700	59,800	197,700	160,000
Direct premiums written	96,600	114,700	245,100	249,800
Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	7,400	7,400	22,100	22,100
Direct premiums written	6,800	6,800	21,700	20,200
Other \| Private passenger automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Homeowners \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Commercial automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	7,400	7,400	22,100	22,100
Direct premiums written	$ 6,800	$ 6,800	$ 21,700	$ 20,200