Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net premiums earned	$ 794,517	$ 790,850	$ 779,321	$ 767,085	$ 760,094	$ 745,520	$ 731,546	$ 720,737	$ 3,131,773	$ 2,957,897	$ 2,796,195
Direct premiums written									3,172,000	3,012,900	2,853,200
Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,435,700	2,308,600	2,203,000
Direct premiums written									2,452,700	2,345,800	2,223,100
Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									414,000	379,700	347,900
Direct premiums written									436,900	402,200	374,500
Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									161,300	144,400	121,800
Direct premiums written									166,100	153,500	135,900
Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									120,800	125,200	123,500
Direct premiums written									116,300	111,400	119,700
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,089,900	2,906,600	2,737,300
Direct premiums written									3,144,700	2,983,100	2,808,900
Property and Casualty Lines [Member] | Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,435,700	2,308,600	2,203,000
Direct premiums written									2,452,700	2,345,800	2,223,100
Property and Casualty Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									414,000	379,700	347,900
Direct premiums written									436,900	402,200	374,500
Property and Casualty Lines [Member] | Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									161,300	144,400	121,800
Direct premiums written									166,100	153,500	135,900
Property and Casualty Lines [Member] | Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									78,900	73,900	64,600
Direct premiums written									89,000	81,600	75,400
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									41,900	51,300	58,900
Direct premiums written									27,300	29,800	44,300
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									41,900	51,300	58,900
Direct premiums written									$ 27,300	$ 29,800	$ 44,300