Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 987,512	$ 926,820	$ 1,950,062	$ 1,842,741
Direct premiums written	1,034,100	972,900	2,052,300	1,926,300
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	666,000	650,800	1,325,300	1,304,600
Direct premiums written	653,800	646,000	1,335,000	1,312,500
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	205,900	169,400	397,900	330,200
Direct premiums written	251,800	207,900	462,100	378,100
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	66,100	64,600	130,600	127,200
Direct premiums written	69,800	66,200	141,400	135,100
Other
Segment Reporting Information [Line Items]
Net premiums earned	49,500	42,000	96,300	80,700
Direct premiums written	58,700	52,800	113,800	100,600
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	980,304	919,515	1,935,722	1,828,330
Direct premiums written	1,025,749	963,418	2,037,134	1,910,024
Property & Casualty \| Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	666,000	650,800	1,325,300	1,304,600
Direct premiums written	653,800	646,000	1,335,000	1,312,500
Property & Casualty \| Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	205,900	169,400	397,900	330,200
Direct premiums written	251,800	207,900	462,100	378,100
Property & Casualty \| Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	66,100	64,600	130,600	127,200
Direct premiums written	69,800	66,200	141,400	135,100
Property & Casualty \| Other
Segment Reporting Information [Line Items]
Net premiums earned	42,300	34,700	81,900	66,300
Direct premiums written	50,300	43,300	98,600	84,300
Other
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,300	14,400	14,400
Direct premiums written	8,400	9,500	15,200	16,300
Other \| Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0		0
Other \| Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0		0
Other \| Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0		0
Other \| Other
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,300	14,400	14,400
Direct premiums written	$ 8,400	$ 9,500	$ 15,200	$ 16,300