Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net premiums earned	$ 806,796	$ 801,205	$ 797,666	$ 789,770	$ 794,517	$ 790,850	$ 779,321	$ 767,085	$ 3,195,437	$ 3,131,773	$ 2,957,897
Direct premiums written									3,248,900	3,172,000	3,012,900
Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,473,800	2,435,700	2,308,600
Direct premiums written									2,480,000	2,452,700	2,345,800
Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									431,600	414,000	379,700
Direct premiums written									469,900	436,900	402,200
Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									171,900	161,300	144,400
Direct premiums written									178,200	166,100	153,500
Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									118,100	120,800	125,200
Direct premiums written									120,800	116,300	111,400
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,160,900	3,089,900	2,906,600
Direct premiums written									3,221,000	3,144,700	2,983,100
Property and Casualty Lines [Member] \| Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,473,800	2,435,700	2,308,600
Direct premiums written									2,480,000	2,452,700	2,345,800
Property and Casualty Lines [Member] \| Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									431,600	414,000	379,700
Direct premiums written									469,900	436,900	402,200
Property and Casualty Lines [Member] \| Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									171,900	161,300	144,400
Direct premiums written									178,200	166,100	153,500
Property and Casualty Lines [Member] \| Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									83,600	78,900	73,900
Direct premiums written									92,900	89,000	81,600
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									34,500	41,900	51,300
Direct premiums written									27,900	27,300	29,800
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									34,500	41,900	51,300
Direct premiums written									$ 27,900	$ 27,300	$ 29,800