Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 833,959	$ 797,666	$ 1,642,043	$ 1,587,436
Direct Premiums Written	886,600	800,600	1,755,600	1,614,900
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	645,600	617,300	1,272,500	1,230,500
Direct Premiums Written	666,000	600,100	1,340,000	1,239,100
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	112,700	108,300	220,500	214,000
Direct Premiums Written	138,000	122,700	253,300	226,500
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	46,700	42,400	91,600	83,700
Direct Premiums Written	49,900	45,100	99,600	88,100
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29,000	29,700	57,400	59,200
Direct Premiums Written	32,700	32,700	62,700	61,200
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	826,200	789,000	1,626,400	1,569,700
Direct Premiums Written	879,400	792,100	1,742,000	1,600,700
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	645,600	617,300	1,272,500	1,230,500
Direct Premiums Written	666,000	600,100	1,340,000	1,239,100
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	112,700	108,300	220,500	214,000
Direct Premiums Written	138,000	122,700	253,300	226,500
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	46,700	42,400	91,600	83,700
Direct Premiums Written	49,900	45,100	99,600	88,100
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	21,200	21,000	41,800	41,500
Direct Premiums Written	25,500	24,200	49,100	47,000
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,800	8,700	15,600	17,700
Direct Premiums Written	7,200	8,500	13,600	14,200
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	7,800	8,700	15,600	17,700
Direct Premiums Written	$ 7,200	$ 8,500	$ 13,600	$ 14,200