Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 811,898	$ 888,776	$ 1,734,471	$ 1,759,021
Direct Premiums Written	828,800	946,100	1,786,700	1,868,200
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	579,700	684,300	1,273,600	1,356,900
Direct Premiums Written	555,000	697,600	1,265,400	1,402,900
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	146,500	123,900	286,000	243,200
Direct Premiums Written	180,700	159,000	328,200	289,900
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	51,100	51,400	106,300	101,000
Direct Premiums Written	55,800	55,800	117,500	110,200
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	34,600	29,200	68,600	57,900
Direct Premiums Written	37,300	33,700	75,600	65,200
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	804,800	881,800	1,720,200	1,744,900
Direct Premiums Written	823,100	938,100	1,772,800	1,853,400
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	579,700	684,300	1,273,600	1,356,900
Direct Premiums Written	555,000	697,600	1,265,400	1,402,900
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	146,500	123,900	286,000	243,200
Direct Premiums Written	180,700	159,000	328,200	289,900
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	51,100	51,400	106,300	101,000
Direct Premiums Written	55,800	55,800	117,500	110,200
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	27,500	22,200	54,300	43,800
Direct Premiums Written	31,600	25,700	61,700	50,400
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,000	14,300	14,100
Direct Premiums Written	5,700	8,000	13,900	14,800
Other Segments [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct Premiums Written	0	0	0	0
Other Segments [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct Premiums Written	0	0	0	0
Other Segments [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct Premiums Written	0	0	0	0
Other Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,000	14,300	14,100
Direct Premiums Written	$ 5,700	$ 8,000	$ 13,900	$ 14,800