Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net premiums earned	$ 921,860	$ 899,304	$ 811,897	$ 922,574	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 3,555,635	$ 3,599,418	$ 3,368,411
Direct premiums written									3,654,200	3,774,600	3,550,900
Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,591,300	2,756,500	2,602,100
Direct premiums written									2,569,000	2,820,500	2,703,600
Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									601,300	514,800	459,400
Direct premiums written									678,900	598,500	524,900
Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									225,600	208,700	190,100
Direct premiums written									240,800	217,300	198,500
Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									137,400	119,400	116,800
Direct premiums written									165,500	138,300	123,900
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,526,753	3,571,042	3,337,916
Direct premiums written									3,624,965	3,742,627	3,524,186
Property and Casualty Lines [Member] \| Private passenger automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,591,300	2,756,500	2,602,100
Direct premiums written									2,569,000	2,820,500	2,703,600
Property and Casualty Lines [Member] \| Homeowners [Member]
Segment Reporting Information [Line Items]
Net premiums earned									601,300	514,800	459,400
Direct premiums written									678,900	598,500	524,900
Property and Casualty Lines [Member] \| Commercial automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned									225,600	208,700	190,100
Direct premiums written									240,800	217,300	198,500
Property and Casualty Lines [Member] \| Other insurance product line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									108,600	91,000	86,300
Direct premiums written									136,300	106,300	97,200
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									28,800	28,400	30,500
Direct premiums written									29,200	32,000	26,700
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									28,800	28,400	30,500
Direct premiums written									$ 29,200	$ 32,000	$ 26,700