Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,236,024	$ 1,034,469	$ 2,402,703	$ 2,039,173
Direct premiums written	1,387,300	1,139,500	2,688,400	2,158,000
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	799,600	674,700	1,553,900	1,335,300
Direct premiums written	847,600	698,500	1,673,100	1,332,800
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	283,600	233,900	550,500	456,400
Direct premiums written	361,900	294,500	664,000	536,100
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	94,200	72,100	183,400	141,200
Direct premiums written	98,800	82,700	202,800	162,700
Other
Segment Reporting Information [Line Items]
Net premiums earned	58,600	53,800	114,900	106,300
Direct premiums written	79,000	63,800	148,500	126,400
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,228,696	1,027,270	2,388,065	2,024,832
Direct premiums written	1,380,511	1,132,216	2,674,986	2,144,453
Property & Casualty \| Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	799,600	674,700	1,553,900	1,335,300
Direct premiums written	847,600	698,500	1,673,100	1,332,800
Property & Casualty \| Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	283,600	233,900	550,500	456,400
Direct premiums written	361,900	294,500	664,000	536,100
Property & Casualty \| Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	94,200	72,100	183,400	141,200
Direct premiums written	98,800	82,700	202,800	162,700
Property & Casualty \| Other
Segment Reporting Information [Line Items]
Net premiums earned	51,300	46,600	100,300	91,900
Direct premiums written	72,200	56,500	135,100	112,900
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,200	14,600	14,400
Direct premiums written	6,800	7,300	13,400	13,500
Other \| Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other \| Other
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,200	14,600	14,400
Direct premiums written	$ 6,800	$ 7,300	$ 13,400	$ 13,500