Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 3,741,948	$ 3,555,635	$ 3,599,418
Direct premiums written	3,907,500	3,654,200	3,774,600
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	2,613,500	2,591,300	2,756,500
Direct premiums written	2,640,700	2,569,000	2,820,500
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	698,100	601,300	514,800
Direct premiums written	801,500	678,900	598,500
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	258,600	225,600	208,700
Direct premiums written	259,900	240,800	217,300
Other
Segment Reporting Information [Line Items]
Net premiums earned	171,700	137,400	119,400
Direct premiums written	205,400	165,500	138,300
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	3,712,910	3,526,753	3,571,042
Direct premiums written	3,877,476	3,624,965	3,742,627
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	2,613,500	2,591,300	2,756,500
Direct premiums written	2,640,700	2,569,000	2,820,500
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	698,100	601,300	514,800
Direct premiums written	801,500	678,900	598,500
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	258,600	225,600	208,700
Direct premiums written	259,900	240,800	217,300
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	142,700	108,600	91,000
Direct premiums written	175,400	136,300	106,300
Other
Segment Reporting Information [Line Items]
Net premiums earned	29,000	28,800	28,400
Direct premiums written	30,000	29,200	32,000
Other | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Other
Segment Reporting Information [Line Items]
Net premiums earned	29,000	28,800	28,400
Direct premiums written	$ 30,000	$ 29,200	$ 32,000