Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 5,075,456	$ 4,274,378	$ 3,952,482
Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	5,075,500	4,274,400	3,952,500
Direct premiums written	5,500,800	4,558,500	4,048,500
Private passenger automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	3,278,500	2,796,100	2,655,700
Direct premiums written	3,395,800	2,841,800	2,598,900
Homeowners \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	1,168,700	957,700	830,600
Direct premiums written	1,367,300	1,119,000	940,300
Commercial automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	378,200	303,900	267,900
Direct premiums written	387,000	346,200	276,800
Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	250,100	216,700	198,300
Direct premiums written	350,700	251,500	232,500
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	5,046,099	4,245,369	3,923,642
Direct premiums written	5,474,292	4,531,942	4,018,057
Property & Casualty \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	5,046,100	4,245,400	3,923,600
Direct premiums written	5,474,300	4,531,900	4,018,100
Property & Casualty \| Private passenger automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	3,278,500	2,796,100	2,655,700
Direct premiums written	3,395,800	2,841,800	2,598,900
Property & Casualty \| Homeowners \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	1,168,700	957,700	830,600
Direct premiums written	1,367,300	1,119,000	940,300
Property & Casualty \| Commercial automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	378,200	303,900	267,900
Direct premiums written	387,000	346,200	276,800
Property & Casualty \| Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	220,700	187,700	169,400
Direct premiums written	324,200	224,900	202,100
Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	29,400	29,000	28,900
Direct premiums written	26,500	26,600	30,400
Other \| Private passenger automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other \| Homeowners \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other \| Commercial automobile \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other \| Other \| Operating Segments
Segment Reporting Information [Line Items]
Net premiums earned	29,400	29,000	28,900
Direct premiums written	$ 26,500	$ 26,600	$ 30,400