Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Net premiums earned	$ 870,245	$ 808,084
Direct Premiums Written	922,100	869,000
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	672,600	627,000
Direct Premiums Written	705,300	674,000
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	119,300	107,800
Direct Premiums Written	130,900	115,300
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	49,600	44,900
Direct Premiums Written	54,400	49,700
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	28,700	28,400
Direct Premiums Written	31,500	30,000
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	863,100	800,300
Direct Premiums Written	915,300	862,600
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	672,600	627,000
Direct Premiums Written	705,300	674,000
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	119,300	107,800
Direct Premiums Written	130,900	115,300
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	49,600	44,900
Direct Premiums Written	54,400	49,700
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	21,600	20,600
Direct Premiums Written	24,700	23,600
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,800
Direct Premiums Written	6,800	6,400
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	7,100	7,800
Direct Premiums Written	$ 6,800	$ 6,400