Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Net premiums earned	$ 888,776	$ 833,959	$ 1,759,021	$ 1,642,043
Direct Premiums Written	946,100	886,600	1,868,200	1,755,600
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	684,300	645,600	1,356,900	1,272,500
Direct Premiums Written	697,600	666,000	1,402,900	1,340,000
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	123,900	112,700	243,200	220,500
Direct Premiums Written	159,000	138,000	289,900	253,300
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	51,400	46,700	101,000	91,600
Direct Premiums Written	55,800	49,900	110,200	99,600
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29,200	29,000	57,900	57,400
Direct Premiums Written	33,700	32,700	65,200	62,700
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	881,800	826,200	1,744,900	1,626,400
Direct Premiums Written	938,100	879,400	1,853,400	1,742,000
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	684,300	645,600	1,356,900	1,272,500
Direct Premiums Written	697,600	666,000	1,402,900	1,340,000
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	123,900	112,700	243,200	220,500
Direct Premiums Written	159,000	138,000	289,900	253,300
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	51,400	46,700	101,000	91,600
Direct Premiums Written	55,800	49,900	110,200	99,600
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	22,200	21,200	43,800	41,800
Direct Premiums Written	25,700	25,500	50,400	49,100
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,000	7,800	14,100	15,600
Direct Premiums Written	8,000	7,200	14,800	13,600
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	7,000	7,800	14,100	15,600
Direct Premiums Written	$ 8,000	$ 7,200	$ 14,800	$ 13,600