Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 868,233	$ 858,135	$ 833,959	$ 808,084	$ 3,599,418	$ 3,368,411	$ 3,195,437
Direct premiums written									3,774,600	3,550,900	3,248,900
Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,756,500	2,602,100	2,473,800
Direct premiums written									2,820,500	2,703,600	2,480,000
Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									514,800	459,400	431,600
Direct premiums written									598,500	524,900	469,900
Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									208,700	190,100	171,900
Direct premiums written									217,300	198,500	178,200
Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									119,400	116,800	118,100
Direct premiums written									138,300	123,900	120,800
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,571,000	3,337,900	3,160,900
Direct premiums written									3,742,600	3,524,200	3,221,000
Property and Casualty Lines [Member] | Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,756,500	2,602,100	2,473,800
Direct premiums written									2,820,500	2,703,600	2,480,000
Property and Casualty Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									514,800	459,400	431,600
Direct premiums written									598,500	524,900	469,900
Property and Casualty Lines [Member] | Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									208,700	190,100	171,900
Direct premiums written									217,300	198,500	178,200
Property and Casualty Lines [Member] | Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									91,000	86,300	83,600
Direct premiums written									106,300	97,200	92,900
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									28,400	30,500	34,500
Direct premiums written									32,000	26,700	27,900
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									28,400	30,500	34,500
Direct premiums written									$ 32,000	$ 26,700	$ 27,900