Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 940,941	$ 899,304	$ 2,783,682	$ 2,633,775
Direct premiums written	1,032,100	957,000	2,958,400	2,743,700
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	652,000	653,000	1,956,600	1,926,500
Direct premiums written	693,900	667,400	2,006,400	1,932,800
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	178,900	151,200	509,100	437,300
Direct premiums written	222,600	183,700	600,700	511,900
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	65,600	57,800	192,800	164,100
Direct premiums written	62,300	62,000	197,400	179,500
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	44,400	37,300	125,200	105,900
Direct premiums written	53,300	43,900	153,900	119,500
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	933,605	892,012	2,761,934	2,612,153
Direct premiums written	1,024,753	949,026	2,934,777	2,721,801
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	652,000	653,000	1,956,600	1,926,500
Direct premiums written	693,900	667,400	2,006,400	1,932,800
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	178,900	151,200	509,100	437,300
Direct premiums written	222,600	183,700	600,700	511,900
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	65,600	57,800	192,800	164,100
Direct premiums written	62,300	62,000	197,400	179,500
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	37,100	30,000	103,400	84,300
Direct premiums written	46,000	35,900	130,300	97,600
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,300	21,800	21,600
Direct premiums written	7,300	8,000	23,600	21,900
Other Segments [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other Segments [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other Segments [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct premiums written	0	0	0	0
Other Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,300	21,800	21,600
Direct premiums written	$ 7,300	$ 8,000	$ 23,600	$ 21,900