Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 858,135	$ 801,205	$ 2,500,178	$ 2,388,641
Direct Premiums Written	917,500	835,100	2,673,100	2,450,000
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	659,600	619,800	1,932,100	1,850,300
Direct Premiums Written	695,100	633,700	2,035,100	1,872,800
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	120,000	108,300	340,500	322,300
Direct Premiums Written	140,700	125,800	394,000	352,300
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	48,700	43,600	140,300	127,300
Direct Premiums Written	50,300	45,600	149,900	133,700
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29,800	29,500	87,300	88,700
Direct Premiums Written	31,400	30,000	94,100	91,200
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	850,500	792,700	2,476,900	2,362,400
Direct Premiums Written	910,600	828,000	2,652,600	2,428,700
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	659,600	619,800	1,932,100	1,850,300
Direct Premiums Written	695,100	633,700	2,035,100	1,872,800
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	120,000	108,300	340,500	322,300
Direct Premiums Written	140,700	125,800	394,000	352,300
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	48,700	43,600	140,300	127,300
Direct Premiums Written	50,300	45,600	149,900	133,700
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	22,200	21,000	64,000	62,500
Direct Premiums Written	24,500	22,900	73,600	69,900
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,600	8,500	23,300	26,200
Direct Premiums Written	6,900	7,100	20,500	21,300
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	7,600	8,500	23,300	26,200
Direct Premiums Written	$ 6,900	$ 7,100	$ 20,500	$ 21,300