Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 899,304	$ 915,012	$ 2,633,775	$ 2,674,034
Direct Premiums Written	957,000	997,000	2,743,700	2,865,200
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	653,000	698,200	1,926,500	2,055,100
Direct Premiums Written	667,400	743,900	1,932,800	2,146,800
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	151,200	133,500	437,300	376,700
Direct Premiums Written	183,700	162,900	511,900	452,800
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	57,800	53,500	164,100	154,500
Direct Premiums Written	62,000	55,200	179,500	165,400
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	37,300	29,800	105,900	87,700
Direct Premiums Written	43,900	35,000	119,500	100,200
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	892,000	907,900	2,612,200	2,652,800
Direct Premiums Written	949,000	987,800	2,721,800	2,841,200
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	653,000	698,200	1,926,500	2,055,100
Direct Premiums Written	667,400	743,900	1,932,800	2,146,800
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	151,200	133,500	437,300	376,700
Direct Premiums Written	183,700	162,900	511,900	452,800
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	57,800	53,500	164,100	154,500
Direct Premiums Written	62,000	55,200	179,500	165,400
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	30,000	22,700	84,300	66,500
Direct Premiums Written	35,900	25,800	97,600	76,200
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,100	21,600	21,200
Direct Premiums Written	8,000	9,200	21,900	24,000
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	7,100	21,600	21,200
Direct Premiums Written	$ 8,000	$ 9,200	$ 21,900	$ 24,000