Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Net premiums earned	$ 1,497,767	$ 1,366,738	$ 2,950,180	$ 2,649,808
Operating Segments
Segment Reporting [Line Items]
Net premiums earned	1,497,800	1,366,700	2,950,200	2,649,800
Direct premiums written	1,623,600	1,485,000	3,196,300	2,930,400
Private passenger automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	917,600	887,400	1,816,200	1,746,000
Direct premiums written	913,500	881,500	1,853,900	1,788,000
Homeowners \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	386,900	316,900	751,400	572,900
Direct premiums written	501,400	434,500	917,800	778,700
Commercial automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	102,200	95,900	199,800	191,200
Direct premiums written	109,400	93,900	229,000	200,400
Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	91,100	66,500	182,800	139,700
Direct premiums written	99,300	75,100	195,600	163,300
Property & Casualty
Segment Reporting [Line Items]
Net premiums earned	1,490,452	1,359,342	2,935,548	2,635,100
Direct premiums written	1,616,772	1,477,169	3,182,902	2,915,636
Property & Casualty \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	1,490,500	1,359,300	2,935,600	2,635,100
Direct premiums written	1,616,800	1,477,200	3,182,900	2,915,600
Property & Casualty \| Private passenger automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	917,600	887,400	1,816,200	1,746,000
Direct premiums written	913,500	881,500	1,853,900	1,788,000
Property & Casualty \| Homeowners \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	386,900	316,900	751,400	572,900
Direct premiums written	501,400	434,500	917,800	778,700
Property & Casualty \| Commercial automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	102,200	95,900	199,800	191,200
Direct premiums written	109,400	93,900	229,000	200,400
Property & Casualty \| Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	83,800	59,100	168,200	125,000
Direct premiums written	92,500	67,300	182,200	148,500
Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	7,300	7,400	14,600	14,700
Direct premiums written	6,800	7,800	13,400	14,800
Other \| Private passenger automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Homeowners \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Commercial automobile \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Other \| Operating Segments
Segment Reporting [Line Items]
Net premiums earned	7,300	7,400	14,600	14,700
Direct premiums written	$ 6,800	$ 7,800	$ 13,400	$ 14,800