Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net premiums earned	$ 868,233	$ 858,135	$ 833,959	$ 808,084	$ 806,796	$ 801,205	$ 797,666	$ 789,770	$ 3,368,411	$ 3,195,437	$ 3,131,773
Direct premiums written									3,550,900	3,248,900	3,172,000
Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,602,100	2,473,800	2,435,700
Direct premiums written									2,703,600	2,480,000	2,452,700
Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									459,400	431,600	414,000
Direct premiums written									524,900	469,900	436,900
Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									190,100	171,900	161,300
Direct premiums written									198,500	178,200	166,100
Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									116,800	118,100	120,800
Direct premiums written									123,900	120,800	116,300
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,337,900	3,160,900	3,089,900
Direct premiums written									3,524,200	3,221,000	3,144,700
Property and Casualty Lines [Member] | Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,602,100	2,473,800	2,435,700
Direct premiums written									2,703,600	2,480,000	2,452,700
Property and Casualty Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									459,400	431,600	414,000
Direct premiums written									524,900	469,900	436,900
Property and Casualty Lines [Member] | Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									190,100	171,900	161,300
Direct premiums written									198,500	178,200	166,100
Property and Casualty Lines [Member] | Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									86,300	83,600	78,900
Direct premiums written									97,200	92,900	89,000
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									30,500	34,500	41,900
Direct premiums written									26,700	27,900	27,300
Other Segments [Member] | Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									0	0	0
Direct premiums written									0	0	0
Other Segments [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									0	0	0
Direct premiums written									0	0	0
Other Segments [Member] | Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									0	0	0
Direct premiums written									0	0	0
Other Segments [Member] | Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									30,500	34,500	41,900
Direct premiums written									$ 26,700	$ 27,900	$ 27,300