Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 922,574	$ 870,245
Direct Premiums Written	957,900	922,100
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	693,900	672,600
Direct Premiums Written	710,400	705,300
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	139,500	119,300
Direct Premiums Written	147,500	130,900
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	55,200	49,600
Direct Premiums Written	61,700	54,400
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	34,000	28,700
Direct Premiums Written	38,300	31,500
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	915,400	863,100
Direct Premiums Written	949,700	915,300
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	693,900	672,600
Direct Premiums Written	710,400	705,300
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	139,500	119,300
Direct Premiums Written	147,500	130,900
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	55,200	49,600
Direct Premiums Written	61,700	54,400
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	26,800	21,600
Direct Premiums Written	30,100	24,700
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,100
Direct Premiums Written	8,200	6,800
Other Segments [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Direct Premiums Written	0	0
Other Segments [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Direct Premiums Written	0	0
Other Segments [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Direct Premiums Written	0	0
Other Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,100
Direct Premiums Written	$ 8,200	$ 6,800