Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting [Line Items]
Net premiums earned	$ 1,452,413	$ 1,283,069
Operating Segments
Segment Reporting [Line Items]
Net premiums earned	1,452,400	1,283,100
Direct premiums written	1,572,700	1,445,400
Private passenger automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	898,600	858,600
Direct premiums written	940,400	906,500
Homeowners \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	364,500	256,000
Direct premiums written	416,400	344,200
Commercial automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	97,600	95,300
Direct premiums written	119,600	106,500
Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	91,700	73,200
Direct premiums written	96,300	88,200
Property & Casualty
Segment Reporting [Line Items]
Net premiums earned	1,445,096	1,275,758
Direct premiums written	1,566,130	1,438,467
Property & Casualty \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	1,445,100	1,275,800
Direct premiums written	1,566,100	1,438,500
Property & Casualty \| Private passenger automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	898,600	858,600
Direct premiums written	940,400	906,500
Property & Casualty \| Homeowners \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	364,500	256,000
Direct premiums written	416,400	344,200
Property & Casualty \| Commercial automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	97,600	95,300
Direct premiums written	119,600	106,500
Property & Casualty \| Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	84,400	65,900
Direct premiums written	89,700	81,300
Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	7,300	7,300
Direct premiums written	6,600	6,900
Other \| Private passenger automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	0	0
Direct premiums written	0	0
Other \| Homeowners \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	0	0
Direct premiums written	0	0
Other \| Commercial automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	0	0
Direct premiums written	0	0
Other \| Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	7,300	7,300
Direct premiums written	$ 6,600	$ 6,900