Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net	$ 915,922	$ 922,574
Direct	953,400	957,900
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net	653,700	693,900
Direct	666,500	710,400
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net	160,800	139,500
Direct	170,200	147,500
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net	62,600	55,200
Direct	68,900	61,700
Other [Member]
Segment Reporting Information [Line Items]
Net	38,800	34,000
Direct	47,800	38,300
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net	908,815	915,345
Direct	946,606	949,745
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net	653,700	693,900
Direct	666,500	710,400
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net	160,800	139,500
Direct	170,200	147,500
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net	62,600	55,200
Direct	68,900	61,700
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net	31,700	26,700
Direct	41,000	30,100
Other Segments [Member]
Segment Reporting Information [Line Items]
Net	7,100	7,300
Direct	6,800	8,200
Other Segments [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net	0	0
Direct	0	0
Other Segments [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net	0	0
Direct	0	0
Other Segments [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net	0	0
Direct	0	0
Other Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net	7,100	7,300
Direct	$ 6,800	$ 8,200